Most Recent Bulletin Report: September 2009 (BGVN 34:09)

Our most recent report on Anatahan (BGVN 33:12) discussed sulfur dioxide emissions and steam plumes during 2008. This report covers activity between January and October 2009.

A team of research scientists from the University of Tokyo and Kyushu University visited the volcano during the week of 19 January. They worked with the Emergency Management Office of the Commonwealth of the Northern Mariana Islands (CNMI) to perform seismic station maintenance. The team observed no unusual volcanic phenomena. Seismic levels remained low, and no anomalies were observed in satellite imagery.

The U.S. Geological Survey (USGS) reported that seismic activity at Anatahan during the first half of 2009 was generally at background levels. On 11 February a brief episode of tremor occurred. A low level plume was observed in satellite images on 13 June, but there was no evidence that it contained ash. Nothing unusual was observed in satellite images throughout the rest of the week. According to the USGS, Anatahan was quiet as of 6 November.

During 7-8 August, small sulfur dioxide plumes from Anatahan were visible on satellite imagery. On 8 August, seismicity decreased significantly. During 8-9 August, steam plumes with small amounts of ash rose to an altitude of 1.8 km (6,000 ft) a.s.l. On 12 August, the Volcanic Alert Level was lowered to Advisory and the Aviation Color Code to Yellow.

During 30 July-5 August seismic tremor from Anatahan fluctuated. Gas-and-steam plumes possibly containing some ash were occasionally visible on satellite imagery at an altitude of 1.5 km (5,000 ft) a.s.l. and below. Plumes drifted W and NW. The Volcanic Alert Level remained at Watch and the Aviation Color Code remained at Orange.

The USGS reported that seismic tremor from Anatahan slowly increased during 16-26 July. A diffuse sulfur-dioxide plume drifted W on 17 July. According to a Washington VAAC report a low-level plume possibly containing ash drifted 50 km NW on 26 July. The Volcanic Alert Level was raised to Advisory and the Aviation Color Code was raised to Yellow. According to another Washington VAAC report on 28 July, an ash plume rose to an altitude of 1.5 km (5,000 ft) a.s.l. and drifted N. Seismic tremor continued to increase. A sulfur dioxide plume drifted NW. The Volcanic Alert Level was raised to Watch and the Aviation Color Code was raised to Orange. During 29-30 July, seismic tremor levels remained elevated, but possibly slightly decreased.

The USGS reported that seismicity at Anatahan remained elevated during 19-25 March. Based on observations of satellite imagery, the Washington VAAC reported that diffuse gas-and-ash plumes drifted SW, NE, and E during 19-22 March. The Volcanic Alert Level remained at Watch and the Aviation Color Code remained at Orange.

The USGS reported that elevated seismicity at Anatahan continued during 12-13 March, then dropped to near background levels on 14 and 15 March. Based on observations of satellite imagery, the Washington VAAC reported that an ash plume was spotted around the summit area on 13 March and later that day drifted W and NNE. On 15 March, a possible diffuse ash plume was visible on satellite imagery drifting W. During 16-18 March seismicity remained low but was punctuated by occasional short bursts (typically about a minute in duration) of increased tremor. The Volcanic Alert Level remained at Watch and the Aviation Color Code remained at Orange.

The USGS reported that levels of seismicity at Anatahan were elevated during 5-10 March, but decreased significantly on 11 March. On 5 March, diffuse ash plumes drifted as far as 120 km NW and 95 km SE. During 5-7 March, continuous ash emissions produced plumes that rose to altitudes less than 3 km (10,000 ft) a.s.l. and drifted NE, SW, and W. On 8 and 9 March, a sulfur dioxide plume was detected by the satellite-based Ozone Monitoring Instrument (OMI) and drifted SW and WSW. According to reports from the Washington VAAC, a diffuse ash plume rose to an altitude less than 3 km (10,000 ft) a.s.l. and drifted W on 10 March.

The USGS reported that levels of seismicity at Anatahan were elevated during 27 February-4 March. During 27-29 February emissions of sulfur dioxide were detected by the satellite-based Ozone Monitoring Instrument (OMI). Based on observations of satellite imagery, the Washington VAAC reported that an ash plume drifted SSW on 28 February. The USGS reported that a second plume rose to an altitude of less than 3 km (10,000 ft) a.s.l. and drifted 240 km NW during 3-4 March.

The USGS reported that emissions of sulfur dioxide from Anatahan were detected by the satellite-based Ozone Monitoring Instrument (OMI) during 20-25 February. Seismicity was elevated during 20-26 February, though levels varied greatly during 23-25 February. The Washington VAAC reported that plumes possibly containing some ash were visible on satellite imagery drifting WSW, SW, E, W, and SE during 20 and 23-24 February. A Volcanic Haze Advisory was issued by the Emergency Management Office (EMO) on 25 February for Tinian, Saipan, and Rota because of elevated sulfur dioxide levels. This advisory was cancelled on 26 February. The Volcanic Alert Level remained at Watch and the Aviation Color Code remained at Orange.

The USGS reported a diffuse plume from Anatahan, possibly containing some ash, was visible on satellite imagery drifting SW on 14 February. On 17 and 19 February seismicity increased. On 18 February, both a low-level steam plume that possibly contained ash and a sulfur dioxide plume were visible on satellite imagery drifting SW. A sulfur dioxide plume was again noted on 19 February. The Volcanic Alert Level remained at Watch and the Aviation Color Code remained at Orange.

The USGS reported that seismic tremor levels at Anatahan were relatively low during 8-13 February, except for short-lived increases during 8-9 and 12-13 February. On 9 February, a diffuse steam plume that possibly contained ash was observed on satellite imagery and drifted W. The Washington VAAC reported that more steam plumes possibly containing some ash were visible on satellite imagery on 11, 12, and 13 February and drifted NW and SE. On 13 February, vog (volcanic fog) was also observed N and W of Saipan. Emissions of sulfur dioxide were detected by the satellite-based Ozone Monitoring Instrument (OMI). The Volcanic Alert Level remained at Watch and the Aviation Color Code remained at Orange.

The USGS reported that elevated seismic tremor levels at Anatahan were detected during the last week in January through 5 February. Observations of satellite imagery showed that the lake in the E crater had disappeared, and steam and sulfur dioxide plumes drifted generally W and SW. On 3 February, an ash plume rose to an estimated altitude of below 2.3 km (7,500 ft) a.s.l. and drifted W. On 5 February, the USGS announced that the Volcanic Alert Level was raised to Watch and the Aviation Color Code was raised to Orange as a result of the observed ash emissions.

The USGS reported that elevated seismic tremor levels at Anatahan detected on 16 January were sustained through 23 January and episodic during 23-28 January. During 20-28 January, a sulfur dioxide plume was detected by the satellite-based Ozone Monitoring Instrument (OMI) and drifted W and SW. A steam plume was also detected on satellite imagery. The Volcanic Alert Level remained at Advisory and the Aviation Color Code remained at Yellow.

The USGS reported that on several days during 12-20 January, sulfur dioxide plumes from Anatahan were detected by the satellite-based Ozone Monitoring Instrument (OMI). Seismic tremor increased on 16 January and remained elevated on 20 January. The Volcanic Alert Level remained at Advisory and the Aviation Color Code remained at Yellow.

The USGS reported that during 29 December-5 January 2008, low-level steaming and sulfur dioxide emission from Anatahan were visible on satellite imagery. Seismicity briefly increased on 2 January and then diminished. A report suggested that the lake level in the E crater had been dropping since September. The Volcanic Alert Level remained at Advisory and the Aviation Color Code remained at Yellow.

Low-level tremor at Anatahan continued during 21-29 December. On 31 December, the Washington VAAC reported that a gas-and-steam plume with low ash content was visible on satellite imagery and drifted NW. The Volcanic Alert Level remained at Advisory and the Aviation Color Code remained at Yellow.

Low-level tremor at Anatahan continued during 14-21 December. On 14 December, the Washington VAAC reported that a steam plume was visible on satellite data.The Volcanic Alert Level remained at Advisory and the Aviation Color Code remained at Yellow.

Low-level tremor at Anatahan continued during 7-14 December. On 10 December, a plume was visible on MODIS satellite imagery that was unconfirmed by field reports. The Volcanic Alert Level remained at Advisory and the Aviation Color Code remained at Yellow.

The amplitude of volcanic tremor at Anatahan gradually increased through October and November and small explosions occurred during the last week of November. On 29 November, the Volcanic Alert Level was raised to Advisory and the Aviation Color Code was raised to Yellow based on the increase in seismicity.

Seismic activity at Anatahan returned to background levels on 1 October and remained low through 12 October. On 12 October, the Volcanic Alert Level was lowered to Normal and the Aviation Color Code was lowered to Green.

Gas-and-steam plumes from Anatahan were visible on satellite imagery when the island was visible through cloud cover during 18 August-15 September. USGS reported that seismicity increased on 9 September and remained elevated through 15 September. On 15 September, the Volcanic Alert Level was raised to Advisory and the Aviation Color Code was raised to Yellow. Seismic activity remained above background levels during 15 September-3 October. During 21-24 September, elevated levels of sulfur dioxide were reported in Saipan.

On 25 March, USGS reported that the Volcanic Alert Level at Anatahan was decreased from Advisory to Normal. Seismicity returned to low levels on 17 March. A gas-and-steam plume that was visible on MODIS imagery on 13 March remained present but diffuse during 17-25 March.

USGS reported that seismicity from Anatahan increased on 24 February and remained elevated through 17 March, occasionally punctuated by higher levels of tremor and increased gas emissions. On 13 March, seismic activity increased significantly; a gas plume that was visible on MODIS satellite imagery suggested increased emissions. The next day the Volcanic Alert Level was increased from Normal to Advisory. On 20 March, low levels of tremor were recorded.

Seismic activity at Anatahan was very low during the previous two weeks (late November through early December). Diffuse steam-and-gas plumes were occasionally visible on recent satellite images or during observation flights. On 7 December, the Volcanic Alert Level was lowered from Yellow to Green.

Based on a pilot report, the Washington VAAC reported that an ash plume from Anatahan reached an altitude of 3 km (10,000 ft) a.s.l. on 29 May and drifted W. Emissions from the E crater of vog (volcanic fog), steam, and a gas plume were visible on satellite imagery at about 1333 and increased prior to generation of the ash plume. A report issued from the Washington VAAC on 30 May at 0535 indicated a faint, low-level gas-and-ash plume extending from the summit.

According to the Air Force Weather Agency (AFWA), on 19 March a hot spot at Anatahan was visible on satellite imagery and vog (volcanic fog) extended 200 km from the island. On 24 March around 1330, seismicity at Anatahan abruptly increased to about twice the background level. The seismicity consisted of low-amplitude tremor and small long-period earthquakes, similar to the seismicity on 17 and 18 March. On the 24th, vog from Anatahan was visible on satellite imagery extending W, then curling N. The plume was estimated to be below 1.2 km (4,000 ft) a.s.l. and no ash or hot spots were visible. Anatahan remained at Alert-level Advisory; Aviation Color Code Yellow (Volcanic activity has increased somewhat, but remains fairly low and is being closely monitored).

During January and February, thin gas plumes from Anatahan were occasionally visible on satellite imagery, but became continuous and slightly more dense during 26 February to 19 March. On 17 March around 2200, seismicity abruptly increased by a factor of nearly two and continued at that level for 2 hours. On the 18th around 1400, seismicity again abruptly increased by a factor of nearly two and continued at that level for about 8 hours before returning to the baseline level prior to 17 March. The increased seismicity consisted of small (M 0-1) long-period earthquakes occurring approximately every minute or so, sometimes reaching two per minute. A total of about 600 such events were detected during 17 and 18 March. Volcanic Ash Advisories were issued by the Washington VAAC, but were cancelled when the plume was determined to contain gas and only insignificant amounts of ash. The Alert Level was raised from Normal; Aviation Color Code Green, to Advisory; Aviation Color Code Yellow around 20 March.

During 24-29 August, eruptive activity continued at Anatahan with ash plumes rising to a maximum height of ~6.7 km (22,000 ft) a.s.l. on the 25th. Volcanic tremor and sporadic long-period earthquakes continued to occur at the volcano. Seismic data were not available after 27 August around 0205 when the seismic station on Anatahan went off line.

During 17-22 August, eruptive activity continued at Anatahan with ash plumes rising to heights of ~7.9 km (26,000 ft) a.s.l. Volcanic tremor and sporadic long-period earthquakes continued to occur at the volcano.

During 10-14 August, eruptive activity continued at Anatahan with ash plumes rising to ~6.1 km (~20,000 ft) a.s.l. Volcanic tremor and sporadic long-period earthquakes continued to occur at the volcano.

During 3-9 August, eruptive activity continued at Anatahan with plumes rising several thousands of meters above the volcano. From 3 to 9 August the National Weather Service at Tiyan, Guam issued multiple volcanic ash advisories for the surrounding islands. According to a news article on the night of 3 August, an aircraft suffered engine trouble in mid-air shortly after taking off from the Saipan International Airport, prompting it to return for an emergency landing. The article noted that the Ports Police said no one was injured in the incident.

During 27 July to 1 August, eruptive activity continued at Anatahan, with plumes rising several thousands of meters above the volcano. On 1 August the National Weather Service at Tiyan, Guam issued a volcanic ash advisory for the islands of Saipan and Tinian. A strong sulfur odor was reported by numerous residents, and ash was observed on the tips of aircraft at Saipan International Airport. According to a news article, flights leaving the airport were delayed due to a lack of visibility caused by volcanic haze.

On 21 July around 0815, an eruption at Anatahan produced an ash cloud that rose to ~15.5 km (~51,000 ft) a.s.l. During the rest of 20-26 July, eruptions produced plumes that rose to a maximum height of ~6.1 km (~20,000 ft) a.s.l. on 26 July.

During 13-18 July, eruptive activity continued at Anatahan, with ash plumes rising to a maximum height of ~11 km (36,100 ft) a.s.l. on 13 July. Volcanic tremor and sporadic long-period earthquakes continued to occur during the report period.

During 6-11 July, eruptive activity continued at Anatahan, with steam-and-ash plumes rising to a maximum height of 6.1 km (20,000 ft) a.s.l. On 6 July beginning at 1730 the amount of tremor at the volcano increased and an eruption produced an ash plume to ~12.2 km (~40,000 ft) a.s.l. During 8-11 July, a thin layer of vog (volcanic fog) extended over much of the Philippine Sea.

On 3 July at 1646 an eruption at Anatahan produced a SSE-drifting plume to a height of ~12.2 km (~40,000 ft) a.s.l. according to Guam Meteorological Office radar. Volcanic fog (vog) briefly drifted S over the islands of Saipan and Tinian. During the rest of 29 June to 5 July, steam-and-ash emissions continued to rise to low levels.

During 22-27 June the Air Force Weather Agency (AFWA) observed in satellite imagery a moderately dense cloud of ash and steam that rose to a maximum elevation of ~3 km (9,800 ft) a.s.l., and drifted W. Additional thin ash and volcanic fog (VOG) were visible to the W and N-NW of the island. On 26 June the AFWA also identified in satellite imagery a dense cloud of ash and steam rising to ~3.7 km (12,000 ft) a.s.l. moving towards the W, and VOG to the W, N and NE of the island. No particular seismic signal was associated with the eruptions. By 28 June the seismicity level dropped by about 80% from the continuously high levels of the last week.

On 19 June at 1525 a brief eruption at Anatahan produced a steam-and-ash cloud that reached a height of ~15.2 km (49,900 ft) a.s.l. Guam Meteorological Weather Office radar showed that the cloud drifted E. No particular seismic signal was associated with the eruption. Two days before the eruption, the amplitude of continuous tremor was relatively high. During the days before and after the eruption, ash reached 3-4.6 km (9,800-15,100 ft) a.s.l. and drifted W.

On 11 June beginning at 1622 there were three explosions at Anatahan. The explosions produced a dense ash cloud that rose to ~13.7 km (44,900 ft) a.s.l., making it the second highest rising plume from an eruption at Anatahan since its first historical eruption in 2003. On 12 June, seismicity was at moderately high levels, with periods of high tremor and frequent small long-period earthquakes. Also, satellite imagery showed an ash cloud at a height of ~3 km (10,000 ft) a.s.l.

A gradual increase in the number of long-period (LP) earthquakes and tremor began at Anatahan on 5 June. Both LP and tremor events peaked during 2230-0030 on 6 June. During the peak in activity, more than 350 LP events occurred. Also, tremor amplitudes briefly reached a new high for the current eruptive activity and an ash column reached ~7.9 km (25,900 feet) a.s.l. On 6 June, tremor amplitudes returned to low levels. During the rest of 1-7 June, ash plumes reached a maximum height of 4.3 km (14,100 ft.) a.s.l.

On 11 May the Air Force Weather Agency (AFWA) reported thick ash rising to 4.2 km altitude (14,000 feet) and moving WNW. The thick ash extended in a triangular shape from the summit 444 km (240 nm) to the WSW through 510 km (275 nm) to the NW. A layer of thin ash at 3 km altitude (10,000 feet) extended beyond the thick ash another 1,000 km (550 nm). A broad swath of VOG extended over 2,200 km (1,225 nm) W nearly to the Philippines and over 1400 km (775 nm) NNW of Anatahan. Although the ash plume diminished over the next few days and was not as thick, it remained significant, rising to 2.4 km (8,000 feet) and extending 370 km (200 nm) WNW on the 13th. Scientific personnel from EMO and the USGS repairing and installing equipment the next day reported hearing a continuous roaring sound from 2-3 km W of the active vent. They also saw ash and steam rising by pure convection, not explosively, to 3 km altitude (10,000 feet).

Activity surged to a moderately high level on 5 May, when an extensive ash-and-steam plume to 4.5 km a.s.l. (15,000 feet) was visible in all directions. Ash extended 768 km N, 130 km S to northern Saipan, and 111 km W. VOG extended in a broad swath from 3,000 km W, over the Philippine Islands, to 1,000 km N of Anatahan. By 9 May harmonic tremor amplitude had decreased to near background levels, with a corresponding drop in eruptive activity. As of 10 May the Air Force Weather Agency was reporting ash to about 3 km a.s.l. (10,000 feet) extending 400 km W and an area of VOG less than half that noted on 5 May.

Harmonic tremor at Anatahan dropped dramatically on 1 May after being at high levels for several days. During 27 April to 1 May, the main ash-and-steam plume rose to ~3 km (~10,000 feet) a.s.l. According to a news article, the volcanic plume from Anatahan reached Philippine airspace on 4 May.

Occasional seismic data from Anatahan revealed that seismicity appeared to increase during 24-25 April. During 20-25 April, a continuous thin plume of ash-and-steam rose to less than ~3 km (~10,000 ft) a.s.l. and drifted more than 185 km from the volcano.

Activity decreased at Anatahan after an eruption beginning on 6 April at 0300 produced an ash plume to the highest altitude in recorded history at the volcano ~15.2 km (~50,000 ft) a.s.l. During 7-11 April seismicity was at very low levels, near background. On the 11th, a steam-and-ash plume rose ~2.7 km (~9,000 ft) a.s.l. and drifted ~280 km WSW.

On 6 April around 0300 an explosive eruption began at Anatahan and produced an ash plume to an initial height of ~15.2 km (~50,000 ft) a.s.l. On 5 April at about 2200 seismic signals began to increase slowly and the Washington VAAC began to see increased ash on satellite imagery. The seismicity peaked on 6 April around 0300. The U.S. Air Force Weather Agency reported an upper level ash plume at ~15.2 km (~50,000 ft) a.s.l. blowing E to SE, and a lower level ash plume at ~4.6 km (~15,000 ft) a.s.l. blowing SW; the plume extended more than 465 km. Earth Probe TOMS data on 6 April at 1046 showed a compact sulfur-dioxide cloud drifting E of Anatahan following the eruption.

On 6 April during 0400 to 0900 the seismicity at Anatahan decreased to near background. The seismicity surged for about 1 hour, with amplitudes about one half those reached during the earlier eruption, and subsequently dropped again to near background.

Prior to the 6 April eruption, during 31 March to 4 April the amplitudes of harmonic tremor varied, reaching a 2-month high on the 3rd. Small explosions occurred every one to several minutes, probably associated with cinder-cone formation. Steam-and-ash plumes drifted ~200 km, and vog (fog composed of volcanic gases) drifted ~400 km at altitudes below ~2.4- 4.6 km (~8,000-15,000 ft).

The third eruption in 2005 at Anatahan apparently began on 21 March when seismicity increased. Seismic amplitudes peaked on the 25th and faded out on the 26th. Near the peak on the 25th, the Air Force Weather Agency detected a hot spot on the island on satellite imagery, and reported an ash plume briefly reaching ~5.8 km (~19,000 ft) a.s.l. The plume height soon declined to below 3 km (10,000 ft) a.s.l., and by near midday on the 27th the plume had transitioned from ash and steam, to steam and vog (fog composed of volcanic gases). On the 27th the plume extended ~240 km (~130 nautical mi) SW.

Volcanic and seismic activity increased at Anatahan during 14-21 March in comparison to previous weeks. During 14-17 March, seismicity increased and steam rose a few hundred meters above the volcano. The inner East crater had been nearly filled with lava flows and lapilli since early January. According to seismic data, a small eruption began on 18 March at 1544. On 19 March the Washington VAAC issued an advisory that an ash plume was visible on satellite imagery below 4 km (13,100 ft) a.s.l. Small explosions that began late on 20 March lasted for 14 hours. No emissions were visible on satellite imagery.

Seismic and acoustic records from Anatahan during 17-28 February showed that a very low level of activity continued at the volcano. Amplitudes recorded during 23-28 February were near levels recorded prior to the onset of the 5 January eruption. NASA MODIS (Moderate Resolution Imaging Spectroradiometer) imagery taken on 28 February showed a faint plume of vog (fog composed of volcanic gas) and steam trending W of Anatahan. The 2003 crater floor was almost entirely covered by fresh lava to a diameter of about one kilometer.

Eruptive activity at Anatahan declined steadily during the week of 14 February to a level that was less than 5% of the peak level attained during the eruption that began on 5 January. Ash eruptions continued, and the 2003 crater floor was almost entirely covered by fresh lava to a diameter of about one kilometer. On 21 February, seismic and acoustic records from Anatahan showed very low levels of activity over the previous 5 days. A MODIS image taken at 0115 on 18 February showed a plume of steam and vog (fog composed of volcanic fumes) extending about ~170 km SW of Anatahan.

During 10-13 February, frequent Strombolian explosions continued at Anatathan, but slowly declined to about 5-10% of the peak levels attained during the current eruption. During the report period, ash rose as high as ~6 km a.s.l.

During 3-7 February, Strombolian explosions continued at Anatahan. On several days ash plumes rose as high as ~6 km a.s.l. Vog (fog composed of volcanic gases) drifted as far as 340 km SSE to Guam, decreasing visibility there to 10 km. According to a news report, residents of the near-by islands of Saipan and Rota complained of health problems from the ash and vog.

As of 30 January Anatahan continued to erupt, with Strombolian explosions occurring about every minute, similar to activity of the previous few weeks. Seismicity during the current eruption reached a high on 26 January, at a level about 20% above previous high levels for the year. After midday on the 27th the explosions became larger but less frequent than before. At about that time, a commercial pilot reported ash to ~3 km a.s.l. and satellite imagery also showed a plume of ash and vog (fog composed of volcanic gases) trailing 65-90 km downwind. On 30 January seismicity levels were about 15% below the peak values of January 26.

The third historical eruption of Anatahan began on 5 January, with explosions occasionally occurring through about 19 January. Near mid-day on 20 January seismicity at the volcano dropped abruptly to near background levels, indicating that explosions had ceased. Degassing may have continued. The apparent cessation of Strombolian activity lasted until late 22 January, when explosions suddenly resumed. The eruption peaked at about 0700 on 23 January. Pilots reported ash to heights of 3-4.5 km. After the peak in activity the explosions decreased somewhat, but were still frequent and strong as of 24 January.

The government of the Commonwealth of the Mariana Islands (CNMI), placed Anatahan Island off limits until further notice and concluded that, although the volcano is not currently dangerous to most aircraft within the CNMI airspace, conditions may change rapidly, and aircraft should pass upwind of Anatahan or beyond 30 km downwind from the island and exercise due caution within 30 km of Anatahan.

During 9-14 January, the eruption of Anatahan volcano stabilized, as explosion signals became larger and generally less frequent than previously observed, averaging a few explosions per minute. Early on 16 January, the eruption suddenly stopped for a couple of hours, then the level of instrumentally recorded activity surged to a new high 50 percent above the previous high. Later on 16 January, the eruption declined slowly for several hours before it stopped again, this time for about 8 hours before it returned to the level of 9-14 January. Early on 18 January, the activity level again surged to its second highest level so far. Then around 1000 activity declined to the level of 9-14 January.

With the current high level of eruptive activity, ash could be in the air out to a few tens of kilometers from Anatahan. The Emergency Management Office has placed Anatahan Island off limits until further notice and concludes that, although the volcano is not currently dangerous to most aircraft, conditions may change rapidly, and aircraft should pass upwind of Anatahan or farther than 100 km downwind from the island and otherwise exercise due caution within 50 km of Anatahan.

The third historical eruption of Anatahan began on 5 January. Occasional, small long-period events were noted as early as 2 January, followed by harmonic tremor early on 4 January, which increased in size through midday 5 January. No large events or explosion signals were associated with the onset of the eruption, which probably occurred middle to late 5 January. Guam tower reported a low plume of thin ash and gas up to ~150 m above the volcano early on 6 January. The VAAC reported a plume at 1225 on 6 January that was 60 km long and 20 km wide blowing westward.

Beginning on 6 January, harmonic tremor gave way to frequent signals of Strombolian explosions. An overflight was accomplished on 7 January by Emergency Management Office personnel, who reported ash rising well above 1.5 km over the volcano. A dome was visible in the crater and bombs were observed rising less than 600 m above the crater.

After 6 January, the amplitude of the explosion signals increased slowly, roughly doubling by 1000 on 10 January, a time when explosions occurred every 3-10 seconds. Later on 10 January, the amplitude of the explosion signals plunged suddenly; by the morning of 11 January the amplitudes again underwent a similar two-fold rise and fall in amplitude.

This ongoing Strombolian eruption was very similar in nature and size to the previous eruption of April-July 2004. As a result, the Commonwealth's Emergency Management Office placed Anatahan Island off limits until further notice and concluded that, although the volcano was not currently dangerous to most aircraft passing nearby, conditions could change rapidly. They noted that aircraft should traverse either upwind of the island, or more than 100 km downwind of the island. Aircraft traveling within 50 km of Anatahan should exercise caution.

Based on a pilot report to the Guam Forecast Office, the Washington VAAC reported that ash from Anatahan was at a height of ~3 km a.s.l. on 2 December. Ash was not visible on satellite imagery, but a hotspot was briefly evident on infrared satellite imagery.

On 27 September, the first long-period seismic events since July 2004 were recorded at Anatahan. Only a few, small events were recorded. Beginning on 12 October, several periods of small, rather regularly spaced long-period events were recorded at intervals of 4 to 15 seconds. This seismicity began several hours after the onset of a series of intense tropical depressions and storms. On 18 October, people in Saipan smelled H2S during very hazy visibility, but no plume was detected on satellite imagery by the Washington VAAC.

During 26 July to 3 August, seismicity at Anatahan remained at very low levels. Interpretations of seismic data suggested that the frequent explosions during the previous week had decreased. Ash plumes probably did not rise higher than 6 km above the volcano.

Seismicity at Anatahan approached the highest levels of the year on 23 July. That day, Strombolian explosions frequently threw mostly coarse material up to hundreds of meters at intervals of tens of seconds to minutes. On 26 July, there were nearly continuous ash-and-gas emissions. By 27 July, seismicity had decreased to very low levels in comparison to the previous 2 months, and seismic signals indicated that the frequent individual explosions that occurred during the previous week decreased greatly in size and number.

Seismicity continued at a high level at Anatahan following a slight decline from near-peak levels on 2 July. Observers correlated the seismic activity on July 9 and 12 with Strombolian explosions that ejected tephra about 100 m above the vent at intervals ranging from a few tens of seconds to minutes. An ash plume extending generally westward a few tens of kilometers downwind from Anatahan below ~3 km a.s.l. was observed by scientists visiting the volcano for much of the week.

On 2 July, the seismicity level at Anatahan rose nearly as high as it was in late April 2004. After the 2nd, seismicity declined slightly. The nature of the seismic signals suggested that Strombolian explosions occurred at intervals of tens of seconds to minutes. The Washington VAAC reported that during clear weather on 3 July, a ~30-km-long ash plume was visible below ~3 km a.s.l.

During 14-16 June, the level of seismicity was significantly lower at Anatahan than during the previous week. After the 16th, seismicity returned to the higher levels seen before the 14th and remained there through 28 June. On the 28th a tropical storm knocked out the last seismic station on Anatahan, prohibiting further monitoring until it is repaired.

Low-level volcanic and seismic activity continued at Anatahan during 9-15 June. During a visit to the volcano on 10 June, Strombolian explosions at an active spatter cone threw volcanic material as high as ~100 m every 10-60 seconds. Around 15 June, the amplitude and number of discrete seismic events appeared to slightly decrease.

During 2-8 June, elevated seismicity at Anatahan consisted of discrete explosion signals. Every few minutes explosions threw volcanic material hundreds of meters out of the crater, and steam and ash likely rose a few hundred meters.

During 26 May to 1 June, elevated seismicity at Anatahan consisted of discrete explosion signals. Every few minutes explosions threw volcanic material hundreds of meters out of the crater, and steam and ash likely rose a few hundred meters.

During 20-25 May, volcanic activity continued at Anatahan. Explosions every few minutes threw volcanic material hundreds of meters out of the crater, and steam-and-ash emissions probably rose several hundred meters above the volcano. On 25 May satellite imagery showed an ash plume extending at least 5 km NW of the volcano.

During 13-17 May, volcanic activity continued at Anatahan. Explosions occasionally threw volcanic material hundreds of meters out of the crater, and steam-and-ash emissions probably rose several hundred meters above the volcano. On 17 May the seismic energy release nearly doubled in comparison to the previous several days, but remained well below the peak level reached on 24 April.

The moderate eruption that began at Anatahan on 24 April continued through 12 May. Seismicity remained at high levels and consisted of discrete explosion signals. Beginning around 5 May explosion signals became less frequent (averaging one event every 2 minutes), but stronger (commonly reaching about M 3). Steam-and-ash emissions continued to rise several hundred meters above the volcano.

Early on 28 April, the level of seismicity at Anatahan increased to its highest level since renewed seismic activity began on 24 April. At this time the lava dome probably increased in size. Seismicity decreased somewhat during the following ~5 days, but M 2-2.5 earthquakes and small explosions still occurred about every minute, and steam-and-ash emissions still rose several hundred meters above the volcano. Local authorities placed Anatahan Island off-limits until further notice. They also concluded that although the volcano was not currently dangerous to most aircraft within the Mariana Islands airspace, conditions could change rapidly and aircraft should pass upwind of Anatahan or more than 30 km downwind from the island and exercise due caution within 30-50 km of the volcano.

Seismicity at Anatahan increased abruptly on 24 April at 1052 to a level that had not been reached since the summer of 2003. Around 0600 on 26 April, seismicity leveled off. Later that day, during two 30-minute observation periods, scientists saw regular puffs of yellow/brown steam and ash emitted at 1- to 2-minute intervals. The rate of emissions virtually matched the rate of seismic events during that time. During 24-26 April, steam-and-ash plumes reached a maximum height of about 600 m above the volcano. According to the Washington VAAC, on 24 April a thin plume from Anatahan was visible on satellite imagery ~1 km above the volcano and extending NW from the volcano. Aircraft were warned to proceed with caution near the volcano.

Seismicity remained at high levels and dome growth continued at Anatahan during the reporting week. Small ash emissions rising to ~1 km a.s.l. were likely occurring and the threat of larger eruptions remained.

On 12 April the presence of a new lava dome at Anatahan within a crescent-shaped crater lake was confirmed by scientists. Fresh ejecta were visible within the lowest reaches of the crater. High levels of volcanic seismicity that began on 31 March continued through 12 April, although it was not quite as high as during its 6-7 April peak. Scientists believed that small steam and/or ash emissions occasionally rose to levels below 1 km, but could increase in height relatively abruptly without much warning.

A seismic swarm began beneath Anatahan on 31 March. During 31 March to at least 4 April, volcanic seismicity was at its highest level since the eruption of May-June 2003, with events as large as M 2.5. The earthquakes were usually followed by tremor-like signals that scientists believed indicated steam and/or ash emissions to altitudes well below 1 km. As of 6 April, the seismic swarm continued to intensify slowly.

During 13-17 February, volcanic seismicity at Anatahan was at low background levels. There were no apparent eruption signals or precursory events. In addition, tremor and seismic-energy release were at very low levels.

After more than 5 months of very low-level seismic activity, long-period earthquakes began at Anatahan on 1 February with a maximum magnitude of 2. On 7 February just before 0600 seismicity peaked, with up to 15 events occurring per hour. It then decreased dramatically, but remained well above the levels during the previous few months. By 8 February the size of the volcanic earthquakes had diminished greatly, but their frequency had increased to as many as one every minute. The amplitude of the low-frequency tremor increased. During 9 and 10 February the volcanic earthquakes had become so small that they were essentially not recorded by the seismic network. The amplitude of the low-frequency tremor remained high, but decreased from its peak level during 5-7 February.

During a flight over Anatahan during the week of 7 September, USGS and Emergency Management Office (EMO) personnel did not see any ash emissions, only low-level steam-and-gas emissions. The floor of Anatahan's crater was covered by sediment-laden water. In East Crater there was an active geothermal system, consisting of mud pots, mini-geysers, and steam jetting from the crater walls. Seismicity at Anatahan was at low levels through 16 September.

As of 11 July, the eruption that began at Anatahan on 10 May continued to wane as shown by decreasing volcanic-tremor amplitudes and observations. On 9 July observations revealed that only white steam was being emitted from East Crater along with a small amount of light-brown fume. During 9-15 July, only faint ash plumes were visible on satellite imagery.

Scientists visiting Anatahan on 2 July found relatively low-level eruptive activity continuing at East Crater. The emitted plumes were ~1 km a.s.l., thin, and contained little ash. Several small vents on the crater floor emitted fine-grained ash and fumaroles were visible on the lower crater walls. Volcanic tremor persisted during 2-3 July at relatively low levels.

The eruption that began at Anatahan on 10 May continued through 26 June, with fine ash emitted from the volcano's East Crater drifting W and SW. During 24-26 June, steam was primarily seen rising to low levels above the volcano, but there were periods when a more ash-laden plume rose to a maximum height of ~2 km above the volcano. The bottom of East Crater was obscured, so scientists could not determine if a lava dome was present. Several earthquakes near Anatahan were recorded during 24-26 June at both the Anatahan and Saipan stations. The largest earthquake occurred on 24 June with a magnitude of 4.9. During the report period, ash was sometimes visible on satellite imagery.

The eruption that began at Anatahan on 10 May continued through 24 June, with the emission of low-level steam-and-ash plumes. On 16 June at 1613 US Geological Survey and Commonwealth Scientific and Industrial Research Organization (CSIRO) personnel saw emissions intensify from mainly low-level steam to a more ash-laden plume that rose to a height of ~2.5 km a.s.l. At this time the seismic amplitude increased from small to large. After 16 June only low-amplitude tremor was recorded by the seismic station on the island. According to the Washington VAAC, ash plumes were occasionally visible on satellite imagery at heights around 2.5 km a.s.l.

As of 17 June, continuous ash-and-gas emissions persisted at Anatahan. US Geological Survey and CNMI Emergency Management Office personnel observed the volcano on 12 June. They noted that a lava dome or flow was visible on the crater floor and ash was emitted from several areas. No sound emanated from the crater, spines were visible on the crater floor, and one source for the convecting ash cloud was located on the E side of the east crater. In addition, the east crater seemed deeper than during the previous visit on 6 June. During 5-12 June, the seismic record only contained banded-volcanic tremor, but on the evening of 12 June long-period (M ~2) earthquakes began to be recorded. An explosion earthquake occurred on 14 June at 0010. Scientists believe the earthquake was associated with an explosion that removed much of the small lava dome, because the dome was no longer seen during an overflight on the 14th. After the explosion, a series of long-period earthquakes occurred at regular 1- to 2-minute intervals until ~1400. During the report period, ash was visible on satellite imagery rising to a maximum height of ~3 km a.s.l.

Emissions of ash and gas at Anatahan, which began on 10 May, continued during 4-10 June. The highest reported ash plume rose to ~7.6 km a.s.l. on 3 June at 2013 according to observations from an aircraft. US Geological Survey and Commonwealth of the Northern Mariana Islands Emergency Management Office (EMO) personnel visited Anatahan on 6 June and repaired the seismic station in the east crater. EMO recommended that the state of emergency for Anatahan be extended due to continued volcanism, and that Anatahan residents be permanently relocated.

Emissions of ash and gas at Anatahan, which began on 10 May, continued during 28 May to 3 June. During most of the report week, eruption clouds rose to heights around 3 km a.s.l. Based on a report from an aircraft, the Washington VAAC stated that ash from a fairly vigorous eruption on 31 May around 1025 rose to ~7.6 km a.s.l. By 2202 there were reports of ash to ~ 6 km a.s.l. The TOMS Volcanic Emissions Group reported detecting significant SO2 emissions from Anatahan during 10-30 May, although data gaps prevented measurements on some days.

Volcanic activity that began at Anatahan on 10 May continued during 21-27 May. On 21 May an ash plume was visible on satellite imagery at a height of about 4.9 km a.s.l. drifting W. Around 23 May, typhoon Chan-hom caused a shift in the prevailing wind direction near Anatahan and ash was transported S over the islands of Saipan, Tinian, Rota, and Guam. Due to the ash, airlines cancelled several flights to and from Saipan and Guam international airports. In Saipan and Guam an ash advisory, and a volcanic haze advisory, respectively, were issued on the 23rd stating that residents should be cautious of lower air quality due to the plume of ash, SO2, and other gases over the islands that morning. By 24 May the plume had drifted away from the islands. According to the Washington VAAC, satellite imagery on 24 May at 1413 showed the ash plume at a height around 5.2 km a.s.l. caught up in the cyclonic circulation of the typhoon moving S, then SE. By 27 May the ash cloud was around 3 km a.s.l. and drifting E.

The eruption at Anatahan that began on 10 May continued through 20 May with less intensity than when it began. The Washington VAAC reported that the ash cloud produced by the eruption drifted W and NW and was at a height of ~5 km a.s.l. on 14 May, and ~4 km a.s.l. on 15 May. As of the 20th, steam-and-ash emissions continued, with the resultant clouds remaining below 3 km and drifting primarily W. According to a news report, parts of the island of Anatahan have been covered by up to ~0.5 m of ash. News reports also stated that by 16 May ash had drifted over the Philippines, posing a threat to aviation in the vicinity.

On 10 May around 1700 an eruption began at Anatahan, a volcanic island which has had no historically documented eruptions. Scientists on a small ship about 10 km away saw an ash plume that eventually rose to ~12 km a.s.l. The eruption occurred primarily from Anatahan's eastern crater and observers did not see lava flows. No precursory activity was recorded (the island of Anatahan is uninhabited and lacks working seismometers) and no signs of the impending eruption were seen by scientists who visited the island on 6 May.

According to the Washington VAAC, an ash cloud was visible on satellite imagery beginning around 1730 on 10 May. The next day around 0655 ash was seen moving in three different directions; WNW at a height around 5.5 km a.s.l., SW around 8.5 km a.s.l., and two separate and smaller ash plumes were drifting SE at heights around 13.4 km a.s.l.

Local authorities issued a special advisory on 11 May stating that ". . . the general public especially fisherman, tour operators and commercial pilots are advised to stay away from the island of Anatahan until further notice from the Office of Emergency Management." As of 13 May ash emissions continued and a hot spot was visible on satellite imagery. Residents of the small island of Anatahan (part of the United States Commonwealth of the Northern Mariana Islands) were evacuated in 1990 after a shallow earthquake swarm. The most recent reported seismicity occurred in 1993.

Bulletin Reports

All information contained in these reports is preliminary and subject to change.

Shallow earthquakes that began 30 March (table 1) were felt and heard on Anatahan Island, and associated with an apparent increase in thermal activity from the younger E cone's crater lake. Felt seismicity remained frequent through 1 April. Observations limited to early morning and around noon yielded reports of 9 shocks, each lasting 5-7 seconds, 31 March-1 April. No felt events were reported 2-4 April. A helicopter overflight on 1 April revealed that the crater lake had become turbulent and had changed from its usual dirty green color to a bluish gray or whitish blue. Fumarolic activity had increased and a rotten egg smell was noted. A new landslide was visible on the SW wall of the active crater. The 23 residents of the island were evacuated 4 April, and had not returned as of mid-April.

Table 1. Earthquakes near Anatahan recorded by WWSSN stations, 30 March-1 April 1990. All events were shallow, but preliminary data did not allow precise depth determinations. Courtesy of the NEIC.

04/1990 (BGVN 15:04)Local earthquakes and strong thermal activity; youngest surge deposits appear no more than a few hundred years oldDownload or Cite this Report

At the request of the Civil Defense office of the CNMI, a team from the USGS and the Hawaii Institute of Geophysics monitored seismicity, deformation, and thermal activity, and investigated the geologic and eruptive history of the island during fieldwork 19-27 April. The following is from their preliminary report.

Geologic overview. "Anatahan, ~9 km long by 3 km wide and elongate along an E-W axis, is topped by a compound caldera of both collapse and explosive origin. The caldera's shape roughly parallels the outline of the island. It can generally be divided into E and W craters; the floor of the E bay is ~250 m below the floor of the main caldera and contains a lake. The W crater essentially comprises the caldera floor and is made up of at least five phreatomagmatic explosion craters. The two highest points on the volcano are peaks at the E and W ends of the caldera (540 and 790 m above sea level, respectively). Slopes dip steeply to the sea from these high points as well as from the caldera walls, at angles averaging about 25°. A number of extra-caldera explosion craters have been identified, especially at the E end of the island.

"The oldest units are interbedded phreatomagmatic ashes and andesite flows exposed at the coast. Most of the flows are plagioclase-phyric and 5-15 m thick, and a number are glassy or partly glassy with well-developed flow lineations. The interbedded ashes and flows dip 20-25° in a generally radial direction from the center of the volcano. At numerous locations along the coast, oxidized cinder layers are exposed, usually accompanied by co-magmatic flows. Along both the N and S coasts, slopes are significantly steeper near the ocean, and ocean-induced mass wasting is undoubtedly the cause. Additionally, however, an E-W-trending set of faults on the S flank has probably contributed to both the steepness of the slopes and the straightness of the coastline.

"The youngest volcanic unit is a light brown to gray phreatomagmatic surge deposit. It is very young, in places plastered on the wave-cut cliffs just above the coastline, or even banked against large talus boulders. This ash blankets all surfaces on the slopes and within the caldera, and has only been eroded from gullies. Its thickness is extremely variable, and isopachs do not give an indication of source location. This deposit appears to be no more than a few hundred years old. Human cultural remains were found under 4 m of base surge deposits, and will be radiocarbon dated to give the deposit's maximum age.

Present activity. "An acid lake and a number of acid pools were present within the E crater. The water in the pools was boiling, turbid due to very fine suspended sediment, and had a pH ranging between 0.7 and 1.2. Water within the larger lake was clear except for areas where upwelling stirred up sediment. Water collected at the edge of one of the upwelling areas was warm and had a pH ranging between 1.2 and 1.9. A large area of vegetation had been killed near the lake and ponds, most likely due to overflows of the hot acid water. The air around the lake was breathable, smelled slightly of sulfur, and probably contained an elevated amount of CO2. Gas had accumulated in one of the water samples that had been collected gas-free. A small explosion crater at the W end of the W bay was observed to be steaming.

"Seismicity was monitored from the village and periodically from the center of the W bay during the 9-day investigation. From 19 until 25 April, daily seismicity consisted of 3-4 distant earthquakes, possibly aftershocks of the 6 April M 7.5 earthquake in the Mariana Trench (centered at 15.27°N, 147.53°E, 32 km depth). Additionally, 3-4 local events of M 3-4 were recorded each day, usually in pairs (figure 1 and table 2). Only one was felt.

Figure 1. Portion of seismogram from the temporary station on Anatahan, 22-23 April 1990. Courtesy of Robert Koyanagi, USGS.

Table 2. Earthquakes recorded on Anatahan by the USGS, 19-27 April 1990, using a revolving drum portable seismograph with 1.0 Hz geophone. Magnitude threshold was about M>0.5 for distances <10 km, and M >3.0 for distances >200 km. Events with distances <40 km may be volcanically related and located within ~10 km of Anatahan at varying depths beneath the island (the 10 km distance also includes Sarigan Island, N of Anatahan). Seismic recording did not begin until late 19 April. Distant events may be aftershocks of the M 7.5, 5 April earthquake in the Mariana Trench. Data courtesy of R. Koyanagi.

"An EDM network and two radial tilt stations were installed and monitored during the investigation. The tilt stations showed no changes. Because of limited helicopter time, every line of the EDM network was not reoccupied every day, but extensions of 6-91 mm were recorded between measurements on 25 and 26 April."

No eruption has occurred, but island residents have not returned. Geologists visited Anatahan 25-26 June to monitor seismicity, deformation, and morphologic changes since previous fieldwork 19-27 April. No local earthquakes (nearer than 30 km) with magnitudes >2.5 were noted during 26 hours of seismic monitoring. However, strong background noise from ocean surf raised detection thresholds by nearly an order of magnitude from April values. Deformation measurements revealed no significant changes in line length since the April survey, although one line spanning the length of the caldera showed a 12-mm extension.

Since April, large shallow lakes had disappeared from the pit crater in the E part of the main caldera, leaving an area of dark brown unvegetated mud that blended into the area of killed vegetation seen 2 months earlier. Eight pits roughly 10-15 m in diameter were observed in the crater, five containing roiling, sediment-laden, steaming water; all eight were present in April, but five had been within the lake. The most vigorous steaming was occurring from an area of sulfur deposits at the base of the crater wall E of the former lake. Steaming at this feature had been somewhat less vigorous in April.

Fieldwork was conducted . . . 28 September-3 October . . . . A regional network of single vertical-component, short-period seismometers was installed, with instruments located on the islands of Pagan, Alamagan, and Anatahan, and a receiving station on Saipan. . . .

A seven-member team of USGS volcanologists visited the CNMI 24 September-6 October at the request of the Office of Civil Defense. The team installed [a seismic station] on Anatahan . . .; data are telemetered to Saipan and recorded at Civil Defense headquarters. Quoted material below is from a report by Richard Moore.

"No significant earthquakes have occurred on Anatahan since installation of the seismic and telemetry system on 29 September. Reoccupation of the EDM network established in April showed small changes in line lengths, in accord with the lack of local seismicity. The lake in the eastern crater . . . was full again on 1 October 1990. The water was discolored, but not boiling."

Felt seismicity 30 March-1 April and turbulence in the crater lake of Anatahan prompted the evacuation of Anatahan Island. The island has remained uninhabited since 4 April.

A six-member team of USGS volcanologists visited the Commonwealth of the Northern Mariana Islands 11-27 May 1992 at the request of the CNMI Office of Civil Defense. The team observed all of the islands in the chain N of Saipan, installed a new seismic station at the base of frequently active Pagan, remeasured existing EDM networks, mapped the geology of Alamagan, sampled fumaroles and hot springs, and collected rocks and charcoal for radiocarbon dating. No volcanoes in the chain erupted during the observation period.

Remeasurement of the EDM network on 22 May showed no significant changes, consistent with the lack of shallow seismicity since September 1990. Boiling hot springs on the eastern crater floor and solfataras at the base of the nearby crater wall had maximum temperatures of 98°C.

An earthquake swarm beneath Anatahan 29 May prompted an announcement on 2 June by the Disaster Control Office placing the islands of Anatahan, Farallon de Medinilla, and Sariguan off-limits until further notice. Boaters and airmen were advised to remain >=80 km from the islands.

The 29 May earthquake swarm began gradually at about 0700 with microearthquakes of approximate magnitude 0.5. Scattered events continued with larger events at 0946 (M 2 and M 3), at 0949 (M 2.5), and at 0951 (M 2). Sometime during 1-2 June, 50 microearthquakes of M 0.5-2.0 occurred within about 30 minutes, most with magnitudes <1. Small flurries continued until 2 June; the last activity consisted of a short burst at 1853-59 with about 20 events of M 0.5-1.5. All of the microearthquakes detected were probably within 10 km of the surface. No harmonic tremor was associated with these swarms. A permanent, continuously recording seismic station on the S rim of the caldera telemeters data to Saipan. None of the seismicity was felt because the island is uninhabited. A temporary station was installed for two days to confirm that the activity was of local origin. Seismicity was continuing as of 12 June, when about 20 microearthquakes were recorded during a 10-minute burst between 0200 and 0300. Magnitudes were in the 0.5-3.0 range and focal depths were shallow.

Although some steaming has been reported from three ponds within the inner sub-crater, such activity is not unusual. Also within the region, a large earthquake (M 6.5) occurred on 6 June at 2323 about 105 km ESE of Anatahan and about 55 km E of Farallon de Medinilla.

Seismicity was continuing in early August, although there were no reports of felt earthquakes. Seismicity then decreased in late August and remained low as of early September. Observations in early September revealed no morphological changes at the volcano. The island was evacuated in March-April 1990 following a shallow earthquake swarm and has remained uninhabited.

An explosive eruption on 10 May at Anatahan marked the first report of activity at the volcano since an earthquake swarm on 29 May 1993 that led to the evacuation of the island (BGVN 18:05 and 18:08). No eruptions had previously been documented in historical time from this small volcanic island in the Commonwealth of the Northern Mariana Islands (CNMI) (figure 2).

Figure 2. Map of the Mariana Islands and outline of the adjacent Mariana Trench. The Commonwealth of the Northern Mariana Islands extends from Rota in the south to Farallon de Pajaros in the north. The island of Anatahan is approximately 9 km long and 4 km wide. Courtesy of CNMI Emergency Management Office.

A group of scientists was near Anatahan on 10 May deploying seismographs for the Margins Mariana Subduction Factory Imaging Project, which is comprised of members from Washington University, St. Louis; Scripps Inst. of Oceanography; and CNMI Emergency Management Office. They passed Anatahan as the eruption was occurring. The island was uninhabited at the time. According to members of the research group who viewed the eruption from about 10 km away, the eruption began on 10 May around 1700. The CNMI Emergency Management Office (EMO) reported that the ash cloud produced from the eruption eventually rose to an altitude of ~12 km (figure 3). During an observational helicopter flight, EMO personnel discovered that the eruption was emanating from the eastern crater (figure 4). They noted that only ash was being emitted, no lava flows were seen, and no explosions were seen or heard. The scientists had visited the island on 6 May and saw no signs of any unusual activity.

Figure 3. Photograph taken on 10 May 2003 of an ash cloud produced from the eruption of Anatahan that began that day. The cloud top is at ~ 4.6 km and emanates from the eastern crater. The view is toward the SW. Courtesy of CNMI Emergency Management Office.

Figure 4. Map of Anatahan showing the deep pit on the eastern side of the summit, which is referred to as the East Crater, and is the source of the eruption that began on 10 May 2003. Courtesy of Scott Rowland, University of Hawaii Manoa.

The Washington Volcanic Ash Advisory Center (VAAC) issued an advisory about the Anatahan eruption stating that an ash cloud was visible on satellite imagery on 10 May at 2232 at an estimated altitude of 10.5 km. One layer of the ash cloud drifted south at a speed of ~65 km/hour, and a lower level at an altitude of ~4.5 km drifted W at ~28 km/hr. By 0655 the next day ash was seen in satellite imagery drifting in three different directions: WNW at an altitude around 5.5 km, SW around 8.5 km, and two separate and smaller ash plumes were drifting SE at altitudes around 13.4 km. At this time, a hotspot was visible on GOES-9 imagery.

On 11 May the CNMI Emergency Management Office, Office of the Director issued a special advisory stating, "Due to this active volcano eruption with high level clouds and [an] ash plume, the general public especially fishermen, tour operators and commercial pilots are advise[d] to stay away from the island of Anatahan until further notice from the Office of Emergency Management." The eruption continued through at least 14 May, when the Washington VAAC issued an ash advisory stating that ash was visible on satellite imagery drifting W of Anatahan at an altitude of ~4.9 km.

The explosive eruption that began on 10 May is the first documented eruption from Anatahan in historical time. There were no residents on the island due to their evacuation following a shallow earthquake swarm in 1990 (Moore and others, 1994), and another in 1993 (Sako and others, 1995). Anatahan is a composite volcano that erupts lavas that are primarily dacitic in composition. It has the largest caldera of the volcanoes in the Commonwealth of the Northern Mariana Islands (CNMI). The presence of this caldera indicates that large explosive eruptions are possible.

Strong activity continued over the next few days (BGVN 28:04), with high ash plumes seen in satellite imagery. The area within ~55 km of the island was also placed off-limits to all boats and aircraft not approved by the CNMI Emergency Management Office (EMO). A smaller but nearly continuous eruption column rose from the E crater of Anatahan for the next several weeks. Activity was continuing in early July, but at low levels.

The EMO invited USGS scientists to provide assistance in tracking the volcano's activity and assessing potential hazards shortly after the eruption began. USGS scientists first arrived in Saipan on 30 May to work directly with EMO officials to install and maintain monitoring equipment and interpret data from overflights and a single seismometer operating on Anatahan. This station became operational on 5 June.

Beginning of the eruption, 10-12 May 2003. On 6 May researchers from Washington University, Scripps Institution of Oceanography, and the EMO aboard the MV Super Emerald deployed a seismograph on Anatahan as part of a joint US-Japan Mariana Subduction Imaging Experiment, which is funded by the National Science Foundation. There were no indications of an impending eruption. During the night of 10-11 May the ship was again approaching Anatahan when scientists observed a tremendous lightning display ahead. As morning broke, they saw a pillar of steam and ash billowing to an altitude of 9 km. The ship had to detour around the island to avoid the ashfall.

Initial reports indicated that the eruption began around 2100 on 10 May. Satellite data interpreted by the Washington Volcanic Ash Advisory Center (VAAC) showed that the eruption appeared to have started by 1730. An ash plume was clearly visible in imagery at 2232, resulting in an advisory being issued to the aviation community at 2300 (1300 UTC). Plume heights were reported to be 10-12 km in the early stages of the eruption, with one ash advisory indicating ash to 13.4 km altitude on the 11th. At times multiple clouds were moving in different directions at different altitudes.

On 13 May Joe Kaipat from the CNMI Emergency Management Office (EMO) and seismologist Doug Weins (Washington University) flew to Sarigan (6.5 km W of Anatahan) to retrieve seismic data from a broadband instrument installed on 6 May. Records from the Sarigan station showed increased seismicity commencing at about 1300 on 10 May. The activity remained very strong for about 36 hours before decreasing.

Activity during 13-30 May 2003. A helicopter overflight on 13 May showed that the island was still erupting, but with less intensity than on 11 May. Large volcanic bombs were observed flying high in the air over the crater region, and the whole W side of the island was covered with ash, including the seismograph site. The village appeared to have 15-30 cm of ash (figure 5). Ash advisories for 13-14 May reported that a dense ash cloud was drifting W away from the island, but that it was not continuous and varied in size. Ash plumes through 17 May generally drifted NW or WNW. The eruption clouds through May after the initial activity were generally below ~6 km.

Figure 5. The village on Anatahan covered with ash, 13 May 2003. The recently deployed seismograph is barely visible in the clearing to the left. Note the ash on the roofs. Courtesy of Doug Weins.

On 18 May the EMO group took an overflight accompanied by David Hilton (Scripps Institution of Oceanography) and Tobias Fischer (University of New Mexico). They reported a rising plume comprised of steam and ash. The cloud was much lighter in color, with a reduced ash component compared to the initial phases of the eruption. Bombs, possibly up to several meters in size, were being tossed into the air; most fell back into the E crater. The ash was being blown W, but most of the ashfall was still on the E side of the island. The team landed on the E side of the island and deployed a PS- 2 seismometer that appears to have recorded earthquakes and some tremor. At that site they found ejecta thought to be from the initial stage of the eruption. The ground/vegetation near and under the ejecta was not scorched. Most of the material appeared to be non- juvenile. The largest fragments were up to 50 cm across. The team heard "booms" coming from the crater.

The ongoing explosive activity excavated a deep crater within Anatahan's E crater. Scientists estimated the inner crater was nearly at sea level by about 20 May; before the eruption, the floor of the E crater was 68 m above sea level. On 20 May the EMO group took an overflight and installed a telemetered seismic station. Pressure waves from detonations in the E crater were felt on the E flank. From a helicopter the team also observed rocks several meters across being thrown up above the E crater rim and falling back into the crater. Ash continued to fall on the western two-thirds of the island and out to sea. The ash cloud size and length was variable during 17-23 May; it continued in general to drift WNW from the island, at times spreading over a wide area.

On 23-24 May, typhoon Chan-hom shifted the prevailing east winds to the S, blowing the eruption column toward Saipan and Guam. Light ashfall resulted in flight cancellations and the closure of the Saipan and Guam international airports. Residents of Saipan reported a rotten egg smell associated with the ashfall. The report from Saipan was that 1-2 mm of ash had fallen on the island.

EMO personnel took an overflight on 27 May and reported that ash cloud heights reached 3 km, significantly lower than during the first few days of the eruption. The ash cloud was more opaque and laden with ash; the color was closer to that of 10-11 May than more recent plumes. The streaming ash cloud, still exhibiting variable size and length, drifted NW and NNW through 29 May.

Fieldwork on 21 May 2003. Hilton and Fischer arrived by ship at Anatahan at approximately 0630 on 21 May. The activity level was similar to that on their visit 2 days earlier. The ship sailed through the ashfall out to the SW side of the island, and continued along the W coast. The W coast was draped in ash; vegetation was completely covered giving the island a gray pallor. They landed at 0815 and spent ~4 hours ashore. A trench through the recent deposits on the beach area exposed a 25-cm section from the present eruptive phase with three main layers. The lowermost layer consisted of ~5 cm of fine-grained ash. Next was a layer ~15 cm thick comprised of accretionary lapilli with some fine ash. At the top was a 5-cm-thick layer that was a mixture of coarser grained ash and angular clasts of scoriaceous material. The abandoned village, where a team led by Patrick Shore (Washington University) was working on the seismic station installed on 6 May, was similarly covered in ash with many buildings having collapsed roofs. Two sections also revealed initial ash, covered by accretionary lapilli, then a mixture of ash and scoriaceous material. Pumice was floating in water-collection vessels by the buildings.

From the ship the scientists set up the COSPEC instrument and started a traverse through the plume around 1330. The telescope was oriented vertically and the ship made a N-to-S transect through the volcanic plume at a distance of ~1.5 km from shore. Sulfur dioxide (SO2) in the plume was recorded immediately. The transect took 50 minutes until no SO2 was being detected. In addition, they sailed through the ash fallout. During the traverse, the volcano erupted every 5 minutes with a deep resonating boom. The width of the volcanic plume was ~6 km and its direction was to the SW. From the COSPEC measurements and wind speed data provided by NOAA, the SO2 flux was estimated to be 3,000-4,500 metric tons/day. As the group sailed away from the island around 1430 there was a very large eruption with a significantly louder "boom" than had been heard previously, followed by a dark billowing ash-laden plume.

MODVOLC Thermal Alerts. Thermal satellite observations of the current eruption of Anatahan provided by the HIGP MODIS Thermal Alert Team (http://modis.higp.hawaii.edu) confirmed that activity was heavily concentrated in the E crater (figure 6). The most recent hot-spot (as of 1700 UTC on 28 May) was observed on 24 May. The large amounts of ash produced during the eruption will have obscured some thermal anomalies from the MODIS sensor. Plumes were clearly visible on MODIS imagery on 14, 21, 22, 25, 26, 28, and 30 May (figure 7). The persistent, long plume from this island volcano was frequently detected in imagery from a wide variety of satellite platforms.

Figure 6. Summary of MODIS thermal alerts detected at Anatahan, 11-28 May 2003. Each dot defines the geodetic location of the pixels flagged by the MODVOLC algorithm (Wright and others, 2002) as containing volcanic hot-spots. However, although the coordinate describes the center point of each pixel, the hot-spots could have been located anywhere in the square boxes (which portray the nominal 1-km pixel size of the MODIS instrument.) The shaded circles denote the absolute limits within which the volcanic hot-spots responsible for the anomalies must have been sited (based on a statistical analysis of long-term hot-spot location stability at other volcanoes). The hot-spot locations are referenced to WGS-84 ellipsoid. Map coordinates are in UTM zone 55 (north). Courtesy of the HIGP MODIS Thermal Alert Team (http://modis.higp.hawaii.edu).

Figure 7. Ash plume from Anatahan (indicated by arrows) visible in MODIS imagery from the Aqua satellite, 0320 UTC on 30 May. Image processed by NOAA with data from NASA. Courtesy of NOAA/NASA.

SO2 data from TOMS. Simon Carn reported that the Earth Probe Total Ozone Mapping Spectrometer (EP TOMS) has observed SO2 and ash emissions from the ongoing eruption. No emissions were detected in the EP TOMS overpass at 0116 UTC on 10 May, several hours before the reported eruption onset. On May 11 a data gap over the Marianas prevented detection of proximal emissions, though a small ash cloud (no larger than ~120 km across) was detected ~500 km ESE of Anatahan at 0027 UTC. Washington VAAC estimates suggested a height of 14-15 km for this cloud. A weak SO2 cloud was also observed, displaced from the ash cloud and centered ~560 km SE of Anatahan. This cloud contained an estimated SO2 mass of ~10 kilotons (kt), but it is suspected to be only the distal end of a larger SO2 plume obscured by the data gap. Diffuse ash was also apparent at least 500 km W of the volcano at 0205 UTC, but no measurable SO2.

The EP TOMS orbit was better placed on 12 May at 0115 UTC. At this time an ash cloud extending ~560 km on its long axis was centered ~570 km W of Anatahan. An SO2 cloud, again displaced from the ash, extended ~1,100 km from a point ~510 km W of the volcano to a point ~700 km SE of it. This cloud contained ~110 kt of SO2. On 13 May a data gap covered the Marianas though ash was detected farther W, with no significant new SO2 evident. On 14 May a low-level SO2 plume appeared to be drifting W from Anatahan.

As of May 30 the Earth Probe TOMS instrument continued to detect significant SO2 emissions from Anatahan. No TOMS data were collected during 15-23 May due to a technical fault on the spacecraft. Thereafter, TOMS detected SO2 clouds in the region of Anatahan on 24 May (~19 kt SO2), 25 May (~23 kt minimum), 26 May (~35 kt), 28 May (~70 kt), and 30 May (~50-100 kt). Data gaps covered the Marianas on other days. Given the persistent ash plume from the volcano reported by the Washington VAAC, these SO2 clouds are presumed to be the product of continuous emissions and not discrete explosive events.

Observations from 20 May-8 June 2001. Anatahan was visited during 20 May-8 June 2001 as part of fieldwork in the Northern Marianas (Trusdell and others, 2001), including helicopter observations on 4 June. At that time line lengths on the Anatahan EDM network were measured and showed no significant changes. Most line lengths exhibited small contractions when compared to the data from the 1994 survey. Deformation appeared to be slowing down with no significant changes. Temperatures were measured for several boiling pots and springs on the floor of the E crater. The temperature of the ponds as well as fumaroles ranged from a minimum of 96.7°C to a maximum of 100.3°C.

Trusdell, F.A., Sako, M.K., Moore, R.B., Koyanagi, R.Y., and Schilling, S., 2001, Preliminary studies of seismicity, ground deformation, and geology, Commonwealth of the Northern Mariana Islands, May 20 to June 8, 2001: U.S. Geological Survey, prepared for the Office of the Governor, the Emergency Management Office, and the Office of the Mayor of the Northern Islands, Commonwealth of the Northern Mariana Islands.

The first historical eruption of the small volcanic island Anatahan began at about 1700 on 10 May 2003 (BGVN 28:04). The island is in the Commonwealth of the Northern Mariana Islands (CNMI) and has been uninhabited since it was evacuated on 29 May 1993 as the result of an earthquake swarm (BGVN 18:05 and 18:08). Shortly after the eruption began, the Emergency Management Office (EMO) of CNMI invited U.S. Geological Survey (USGS) scientists to provide assistance in tracking the volcano's activity and assessing potential hazards. This report discusses a seismically detected phreatic stage during 10-20 May followed by a new lava discharge on 4 June. After mid-July seismicity and volcanism declined.

Records from a broadband seismograph installed by Washington University 6.5 km W of the Anatahan crater on 6 May 2003 were retrieved on 20 May. F.A. Trusdell and R. White (USGS) reviewed the records and plotted estimate of the numbers of volcano-tectonic (VT) events and their maximum magnitudes, and an estimate of the background and/or tremor level for 9-11 May 2003 (figure 8). No VT events or tremor appeared during 6-9 May. Unrecorded precursory activity may have occurred prior to the seismograph installation. Trusdell and White described their findings as follows in their report of 5 June 2003.

"Beginning on May 10, the number of hourly events increases from 0-1 the first couple of hours to 20-33 by [1400 and 1500 hours local time]—then surges to more than 100 events/hr beginning at [1600]. These events are all VTs, with impulsive P and S phases that decay rapidly. The largest events didn't exceed about M 2 on May 10. The spectra are broadband with dominant frequencies between 8 and 10 Hz, the higher frequencies probably attenuated by the 6.5 km travel path (the distance between the crater and the Washington University seismograph). Note that late on the 10th, the number of events begins to decrease rapidly and about this same time, the amplitude of the largest events increases rapidly. The largest VT of all (through the last record available, of May 20) had a magnitude of about M 3.1 and occurred a little after noon on the 11th. After that event, both the numbers and amplitudes of the VTs dropped off rapidly (and remained very low through the last record available, on May 20.

"The background noise level remained very low until [1700] on May 10, when the level increased by 2.5x and by [1800] the tremor level was ~6x above the background level. We infer that the rising tremor level corresponds to the approximate onset of gas and ash emission into the atmosphere. The [Washington VAAC (Volcanic Ash Advisory Center)] estimates that the ash first appeared about [1730]. The tremor level increased further and peaked on May 11.

"Notes on May 11-20: The tremor level remained very high for a couple of days before decreasing by about half (10 arbitrary units on the figure) by May 20. By about May 15, unambiguous LP's [(long-period events)] begin to appear. By May 19 and 20, there appear brief moments (several seconds) when the tremor level drops to near background immediately prior to the largest LP's which often contain air phases. Even the largest LP's are not particularly large, maybe M 2-2.5. All of this activity from May 10-20 is compatible with an aggressive phreatic stage."

The spiny surface of a lava flow was first observed in the inner crater on 4 June. The flow appeared to form a mound-shaped lava dome, but its volume is unknown. Scientists also noted the presence of new fault scarps and slump features within the E crater, as well as additional faulting to its W. Such features commonly develop around active vents due to the rise and subsequent eruption of magma. On 5 June the EMO seismic station was repaired and ash samples were collected from the site. Through 12 June, the seismic records showed only continuous ground shaking (tremor) to varying degrees. The most intense periods of tremor lasted 3-10 hours and occurred about every 24-36 hours. On 12 June, three LP earthquakes were recorded, the largest about M 2. Other earthquakes followed on the late afternoon and early evening of 13 June.

Two strong explosions on 14 June removed much of the small new dome in the inner crater. Just before noon on that day, earthquakes began to occur every 1-2 minutes. For the next two days, several episodes of intense tremor and earthquakes lasting ~1.5 hours occurred about every 12 hours. These episodes of increased seismic activity accompanied strong ash emissions from the E crater, with eruption columns higher than 2 km. Quiet intervals in which the eruption column consisted of little ash were accompanied by continuous low tremor. At 1613 on 16 June observers noticed that the light-colored, steam-dominated, eruption cloud got darker and rose very quickly (20-40 seconds) to ~2.5 km altitude (figure 9). At this time, the seismic amplitude went from "small" to "large" (a 5-10x increase). Since 16 June seismic activity has consisted only of low-level tremor, and even that low level was gradually shrinking.

As of 9 July the eruption continued to wane, as shown by decreasing amplitudes of volcanic tremor. Observations from a helicopter on that day revealed only white steam low in the E crater and a minor amount of light brown fume without ash emission.

Anatahan erupted on the evening of 10 May 2003 (BGVN 28:04). The volcano, which forms the uninhabited Anatahan Island in the Commonwealth of the Northern Mariana Islands (CNMI), had no recorded historical eruptions. This report provides observations from a 25 July 2003 report (updated 31 July 2003) by the University of Tokyo Earthquake Research Institute (ERI) documenting fieldwork by their team during 16-19 July 2003. During the inspection, the volcano was quiet, with only weak steaming at the active crater. Seismicity reported by the Commonwealth of the Northern Mariana Islands (CNMI) Emergency Management Office continued into early August.

Tephra deposits. The recent eruption left recognizable tephra deposits consisting mainly of pumice-bearing brown ash in a lower unit and fine gray ash in an upper unit (figure 10). Both the upper and lower units consist of many sub-layers. At the village (NW end of the island) the total thickness of brown ash was 20 cm and gray ash was 3 cm.

Figure 10. Section of tephra seen just S of Anatahan's active crater on 18 July showing deposits laid down in the eruptions that began in May 2003. The section contains a lower (brown) pumice-fall deposit (~ 25 cm thick) covered by multiple layers (~ 20 cm thick) of gray ash from phreatic eruptions. Courtesy of S. Nakada, University of Tokyo.

At the SE part of the island tephra deposits were less than 3 cm thick. Although grass and trees did not show heat damage, plastic bottles had melted. The outer S slope of the active crater in the E caldera was thickly covered by gray ash. Many rills and gullies developed on these deposits due to the impermeable nature of the gray ash, which typically consisted of very fine particles. Occasionally the observers noted partly broken, stripped trees on the slopes, with a thick cover of gray tephra accumulated on the side facing the active crater. Tephra was ~20 cm thick near the crater rim and pumice-bearing tephra below was ~25 cm thick. The latter included blocks and fragments of pumice.

Inside the W caldera, tephra deposits reached a thickness of up to 1 m. Gray ash was deposited most thickly NW of the crater. Pumice-bearing tephra was thickest in the WSW direction from the crater. The latter is consistent with the drift direction of eruption plumes in the earliest stage shown by satellite images (BGVN 28:06). Although most of the trees had survived falling pumice early during the eruption, they were toppled by the strong lateral movement of gray ash during the phreatic phase.

Crater observations. The mid-July fieldwork included two days of helicopter inspection; observers saw only steaming at the active crater. That crater occupied the S part of the E crater, which lies inside the E caldera. The S wall of the active crater extended directly into the wall of the E crater. The new crater was ~300 m across and ~100 m deep, with the deepest part in the S containing a dried-out mud pool.

A mound-like but rugged-ridged lava dome protruded along the active crater's inner N periphery (figure 11). The surface of this recently erupted dome lay beneath a thick cover of gray ash associated with the phreatic eruption. Infrared camera images indicate that it remained at higher temperature than deposits outside the crater.

Figure 11. Aerial view showing the steaming crater at Anatahan from the NW on 19 July 2003. The lava dome (center left) lies inside the crater. A pyroclastic cone had developed on the N side, surrounding the crater. Courtesy of S. Nakada, University of Tokyo.

The dome may have been broken by explosive eruptions in mid-June when high seismic and visual activities were reported. Products of a reamed-out dome may have been broken into small clasts, widely dispersed, and buried by later deposits. On the other hand, neither bombs nor blocks were clearly visible on the floors of either the E crater (outside the pyroclastic cone) or in the E caldera. Thus, the absence of large blocks of lava dome around the active crater could suggest that the original dimensions of the lava dome may have been small and that the dome had undergone comparatively little sculpting by later explosions.

A low pyroclastic cone developed on the crater's N side (figure 11). The maximum thickness of newly deposited tephra exposed in a gully through this cone reached ~20 m.

Chemistry and degassing of magma. Pumice from this eruption was crystal-poor and light to dark brown in color. A pumice block with a light-brown crust and dark-brown vesicular core collected from the pumice-fall layer just S of the active crater was analyzed by x-ray fluorescence spectroscopy at ERI. The crust and core parts were separately analyzed; each contained 61 weight percent SiO2.

Observers saw blue- to purple-colored gas escaping the active crater and smelled a strong rotten-egg near the S rim of the E caldera on 18 July. Instrumental concentration estimates measured 2-4 ppm SO2 and 0.5 ppm H2S. The SO2 emission rate remained moderate to low throughout the inspection; the total SO2 flux was probably less than several thousand tons a day, similar to that at Sakurajima, Japan.

Ongoing activity, July into early August. According to CNMI reports, volcanic tremor and other seismicity at Anatahan persisted through July and into August 2003 at a relatively low level. On 1 August the Anatahan seismic station registered a small swarm of a dozen or so long-period (LP) events of approximate magnitude 1; similar swarms occurred on 4 and 5 August. Several hundred small (LP) events occurred during 5-6 August. The number of small LP events was greater than that of previous days, but the overall energy release appears not to have increased significantly. No LP events were recorded on 7 August.

The first recorded historical eruption at Anatahan, which began on 10 May 2003, continued through that month with nearly continuous ash plumes (BGVN 28:04 and 28:05). Two strong explosions on 14 June removed much of a small lava dome that had been extruded in the crater; dark ash plumes were last reported on 16 June, after which time seismicity decreased significantly (BGVN 28:06). Only steaming without ash emissions was reported by scientists doing fieldwork immediately afterwards (BGVN 28:07) and on overflights in July. Volcanic tremor and other seismicity reported by the Commonwealth of the Northern Mariana Islands (CNMI) Emergency Management Office (EMO) persisted into early August at a relatively low level. This report covers observed activity from 4 August to 5 October 2003.

Seismicity was low throughout the report period and no apparent eruption signals or potential precursory events occurred. Tremor and seismic energy release were at low levels. During 2-6 August, small long-period (LP) events occurred regularly. At the end of that interval, the number of small LP events increased to several hundred in 24 hours, compared to a couple dozen per day earlier in the swarm, but the overall energy release increase was not significant. No LP events were reported again until mid-September. On 5 September, tremor and seismic energy release were reported to be at their lowest levels since early July.

Overflights of the volcano were made by USGS and EMO personnel on 30 August and 8, 9, and 11 September. Observations on these days revealed no ash emissions, and the feeble plume was dominated by steam and lesser amounts of volcanic gases, mainly SO2. Sporadic emissions sometimes rose above the crater rim. The E crater floor was covered by dirty, sediment-laden, steaming water, and an active geothermal system had mud pots, mini-geysers, and steam jets. Steaming water and sulfurous gases were emitted from the crater walls and floor. Observations during an 18 September overflight were similar to those earlier in the month, although the crater floor appeared to be covered by muddy water instead of a shallow lake. A distinct odor of SO2 and blue fume were noted during a helicopter inspection of the E crater lake on 27 September. On 29 September, geysering was seen and the odor of H2S was present in addition to SO2.

By 12 September USGS and EMO had reestablished the original, pre-eruption Anatahan seismic station (ANAT) on the SW caldera rim. On 15 September, several, small-amplitude, LP events lasting up to 15 seconds were visible on the ANAT records with dominant frequencies of 4-5 Hz. Some of the larger events had a short burst of 6-7 Hz energy about 2.5 seconds after the onset. The largest events were barely above background at the E Anatahan station (ANA2) and may have been occurring undetected for the past several weeks. The LP events at the ANAT station continued over the next two days at a rate of several per hour.

The first recorded historical eruption at Anatahan began on 10 May 2003. Activity through early October 2003 was [previously] reported [(BGVN 28:04, 28:05, 28:06, and 28:09)]. This report, in large part contributed by scientists of the Commonwealth of the Northern Mariana Islands Emergency Management Office (CNMI/EMO) discusses Anatahan during October 2003 to early May 2004. During October-February, Anatahan's tremor and volcanic seismicity were consistently low, with no apparent eruption signals or precursory events. Later, in March, April, and May clear emissions began, and in April-May 2004 small dome extrusions occurred. There were several peaks in seismicity, such as 2-8 February and shorter episodes on 13-14 February. Much larger peaks in seismicity, the largest in 2004, took place in late April. Many details on the dome extrusions remain undisclosed; however, near the end of this report there are a variety of data from several contributors, including one clear Modis alert (28 April) and a VAAC reports based on satellite observations of a thin plume (24 April).

During overflights in early October 2003 observers saw no ash emissions. The crater vents continued to emit steam and SO2, and the floor of the crater was in great flux. During periods of rain, the crater floor was covered by sediment-laden water and (or) debris flows. The E crater continued to have an active geothermal system that consisted of mud pots, mini-geysers, and steam jets from the crater walls. In general, emissions continued at a low level.

During early November 2003, regional seismicity was low. For the week of 9-15 December, numerous tiny long-period earthquakes (LPs) were recorded only on the station near the crater, at a rate of ~ 1 every 10 sec. All of these LP events were much smaller than M 0.5, with dominant frequencies of 4-5 Hz. After several months of only very low magnitude LP events, on 1 February 2004 larger ones again began to occur. After increasing slowly in size, they reached M ~ 2, and they took place several times per hour.

Anatahan's seismicity peaked just before 1600 local time on 7 February 2004, with a swarm consisting of up to 15 events occurring per hour. The seismicity then decreased dramatically but remained well above levels of the previous few months. By 1000 on 8 February the maximum magnitude of volcanic earthquakes had diminished greatly from their peak magnitude two days before, but the earthquakes were occurring more often, sometimes as frequently as 1 per minute. The amplitude of low-frequency tremor had also increased considerably. The combined effect was that the level of seismic energy release around 7-8 February, averaged over hours, remained nearly constant at its peak.

By 9 February the magnitude of the volcanic earthquakes decreased significantly and were no longer visible on the records. The amplitude of low-frequency tremor remained high but decreased some from its peak on 8 February. The total daily seismic energy release decreased somewhat from its peak during the previous few days. From 10 February to 29 March 2004, Anatahan volcanic seismicity, tremor, and energy release were all very low, with no apparent eruption signals.

Volcanic seismicity occurred again 13 February during a 6-hour period, the first such seismicity since the episode of 2-7 February. Short tremor episodes began at 0543 and occurred every 8-12 minutes until 1130 on 14 February, the largest being approximately equivalent to a magnitude M 2 earthquake. From 15 February through 30 March 2004, Anatahan tremor and volcanic seismicity stood at low background levels, with no apparent eruption signals or precursory events.

A seismic swarm began beneath Anatahan Island on 31 March, the third such swarm since the eruption of May-June 2003. The largest earthquakes in the swarm were all smaller than those that occurred during the previous swarm in early February 2004. During 2-3 April the swarm intensified significantly. Most earthquakes were followed by long, tremor-like signals that CNMI scientists believed indicated small emissions of steam, possibly bearing ash and thought to rise to altitudes much less than 1,000 m, though they had no visual confirmation of such emissions at the time.

On 6-7 April, Anatahan volcanic seismicity was the highest since the eruption of May-June 2003, with events as large as M ~ 2.5 usually followed by tremor-like signals. That swarm's level of seismic activity remained high through 23 April.

With regard to the rise in seismicity during early 2004, Scientists of the Commonwealth of the Northern Mariana Islands Emergency Management Office (CNMI/EMO) opined that these events were likely the result of magma degassing and/or moving beneath the recently active crater. The Office of the Governor, CNMI, placed Anatahan Island off-limits and concluded that, although the volcano was not currently dangerous to aircraft, pilots should exercise due caution in Anatahan's vicinity.

Dome, ejecta, and lavas. On 12 April, the presence of a new, rather flat lava dome within a crescent-shaped crater lake was confirmed, as was the occurrence of fresh ejecta within the lowest reaches of the crater. The Hawaii Institute of Geophysics and Planetology (HIGP) web site for MODIS hot-spot satellite imagery showed a thermal anomaly for the volcano on 12 April at 1545 UTC. High seismicity through 23 April suggested continuing effusion of relatively small volumes of new lava onto the crater floor.

At 1052 on 24 April, Anatahan's seismic activity increased abruptly, rising to levels unseen since summer 2003 (3 to 4 times as high as those observed in April 2004). About that time a low-level eruption began producing steam and ash to ~ 600 m and an overflight reported incandescent cracks in a fresh lava flow or dome within the inner crater. The presence of a "cow-pie-shaped" dome within the inner crater was verified later.

The seismicity level increased slowly and fairly constantly on 24-25 April to a level similar to that of the eruption of mid-June 2003. During an overflight on 26 April between 1030 and 1100, Juan Camacho (CNMI/EMO) and Erik Hauri (Carnegie, Margins group) observed regular puffs of yellow-brown steam and ash every 1-2 min, a rate almost identical to that of seismic events recorded during that time. The maximum height of the steam and ash plume was estimated at ~ 600 m.

According to the Washington Volcanic Ash Advisory Center (VAAC), at 0725 UTC on 24 April 2004 a thin plume from Anatahan was visible on satellite imagery ~ 1 km above the volcano and extending ~ 460 km NW; this plume was not observable on satellite images taken at 1502 UTC, later in the same day. The HIGP web site for Modis hot-spot satellite imagery showed a thermal anomaly for the volcano on 28 April at 1545 UTC. On 28 April the seismicity level increased still further, a level ~ 25% more energetic than the previous high of 25 April 2004.

In accord with the elevated seismicity, the dome increased in size, and explosions also apparently increased in size and rate. The seismicity then slowly decreased ~ 25% over the next 5 days until 3 May, when it dropped off suddenly but smoothly by another 40%. Subsequently, over several days, the seismicity recovered somewhat to ~ 50% of the highest level of 28 April. On 5 May 2004 small explosions continued to occur every minute or two, and steam and ash still rose hundreds of meters.

The first recorded historical eruption at Anatahan Island began on 10 May 2003 (BGVN 28:04-28:06 and 28:09). More volcanism accompanied increased seismicity beginning 30 March 2004 (BGVN 29:04). Lava was noted in the crater on 15 April. During an overflight on 24 April scientists reported fresh lava within the inner crater. Seismic activity increased abruptly at 1052 on 24 April, escalating to levels higher than recorded since summer 2003, and a moderate eruption initially produced a light ash cloud that rose to altitudes below 2 km. The cloud persisted for only a day or so.

The seismicity level increased further on 24 and 25 April. On 26 April, a flat-shaped dome was observed within the inner crater. On the evening of April 28, the seismicity level peaked, then decreased slowly to about 40% of its peak value by 29 May. That seismicity resulted from strombolian bursts every minute or so that ejected material some hundreds of meters out of the crater, and steam and ash to several hundred meters. After a two-day-long decrease, the seismicity surged on 30-31 May to double the value of the previous few days, resulting from more frequent small explosions (occurring every few tens of seconds) as well as increased tremor.

On 7 and 8 June a 100-km-long, light-colored plume of steam and ash blew W. This was reported by the U.S. Air Force Weather Agency based on Defense Meteorological Satellite Program (DMSP) satellite images (figure 12).

Figure 12. Visual (0.3 nm) image of plume from Anatahan volcano taken from Defense Meteorological Satellite Program satellite on 7 June 2004 at 2139 hours GMT (8 June 2004 at 0739 hours local time). Note that the plume length at this time, measured by the U.S. Air Force Weather Agency, was ~104 km (~56 nautical miles). Courtesy of Charles R. Holliday, U.S. Air Force Weather Agency.

Juan Camacho of the Commonwealth of the Northern Mariana Islands Emergency Management Office (CNMI/EMO) visited the island on 10 June and reported an active spatter cone, from which continuous strombolian explosions threw material as high as 100 m every 10 seconds to one minute. By 15 June, the amplitude and number of discrete events appeared to have decreased slightly.

The current eruption began after increased seismicity on 31 March 2004 (BGVN 29:04 and 29:05). Lava was noted in the crater on [12] April and may have extruded for a few weeks thereafter. The most energetic phase occurred during 24-28 April, when a ash clouds rose ~ 1,000 m. This report summarizes activity from mid-June through September 2004.

During June, seismicity was higher as a result of more frequent small explosions every few tens of seconds, and a 100-km-long, light-colored plume of steam and ash was occasionally visible. A cone active since at least 10 June produced continuous Strombolian explosions that ejected material as high as 100 m every few tens of seconds to minutes, filling the inner crater by about 10 July. The seismicity level was generally significantly lower during 14-16 June, after which it returned to earlier levels. On 27 June a tropical storm knocked out the last seismic station on the island.

The last time the weather was clear enough to see well, on 3 July, the Washington VAAC reported a 31-km-long ash plume well below 3 km altitude. During a few days in mid-July, prolonged periods, 10-15 minutes long, of continuous ash emission began to occur and became more frequent. A plume of light ash and steam trailed tens of kilometers WSW at altitudes less than 3 km.

Seismicity increased on 23 July to approximately double the level of the previous day, approaching the previous high of late April. The seismic signals indicated somewhat larger and longer periods of ash emission. Strombolian explosions continued to occur very frequently. The explosions threw mostly coarse material upward a hundred meters or so at intervals of tens of seconds to a few minutes. A plume of light ash and steam trailed a few tens of kilometers downwind, generally W, at altitudes below 1.8 km. An Air Force Weather Advisory on 24 July reported that their satellite could see the source as a hot spot. The Commonwealth of the Northern Mariana Islands Emergency Management Office (CNMI/EMO) reported a plume of light ash trailing a few tens of kilometers WSW at altitudes below ~ 3 km.

Joe Kaipat (EMO) visited the island on 25 July with personnel from Fish and Wildlife and reported observing an ash plume probably a few tens of kilometers long moving NE. This plume persisted until 3 August, less than a few kilometers long and below 600 m. After 26 July seismicity decreased to a very low level, with the signals indicating that the frequent individual explosions of several days before had decreased significantly in size and number and finally ceased by 31 July. Instead, ash and gas were being ejected almost continuously. Seismicity remained very low and frequent individual explosions ceased through August and September 2004.

Although the latest eruptive period ended in late July (BGVN 29:08), the volcanic system at Anatahan continued to exhibit unrest in the following months. On 27 September, several hours after the onset of a series of intense tropical depressions and storms, the first long-period seismic events since July 2004 were recorded. However, only a few, small events occurred. Beginning on 12 October, several episodes of small, regularly-spaced long-period events were recorded at intervals of 4-15 seconds. On 18 October, people in Saipan smelled H2S during very hazy visibility, but no plume was detected on satellite imagery by the Washington Volcanic Ash Advisory Center (VAAC).

Based on a pilot report to the Guam Forecast Office, the Washington VAAC reported that ash from Anatahan was at a height of ~ 3 km altitude on 2 December. Ash was not visible on satellite imagery, but a hotspot was briefly evident on infrared imagery.

Eruptions in January 2005. Major eruptive activity at Anatahan resumed on 5 January 2005, preceded by two days of small long-period earthquakes and a day of harmonic tremor. The airport tower at Guam confirmed that a plume of diffuse ash and gas up to ~ 200 m above the vent was visible at first light on 6 January, and at the 1225 hours VAAC reported a plume 60 km long and 20 km wide, blowing W.

Frequent Strombolian explosion signals began on 6 January and continued during 7 and 8 January, accompanied by a change in the seismic signals, from harmonic tremor to a broader band tremor, with explosions recorded by microphones several times per minute. The eruption type and activity level were both very similar to the peak eruptive activity during the eruption of April-June 2004 (BGVN 29:04-29:06). During an overflight on 7 January, personnel from the Emergency Management Office (EMO) reported ash rising well above 1,500 m and a plume that likely extended up to 100 km downwind. A dome was visible in the crater and bombs were observed rising less than 600 m.

On 7 January ash rose to ~ 3 km and bombs a meter or more in diameter were expelled to ~ 100 m and formed a new cinder cone ~ 120 m in diameter. The amplitude of the explosion signals increased slowly after 6 January to about double these values by noon on 10 January, with explosions every 3-10 seconds. Explosion signals amplitudes then plunged suddenly to half the values at the start of 10 January. The amplitudes surged again, nearly doubling by approximately 0400 on 11 January, dropping to half the value again by about noon. The eruption apparently stabilized at that level through 14 January. During 15-19 January, the eruption appears to have stopped twice for a few hours but swiftly resumed at higher levels. National Oceanic and Atmospheric Agency (NOAA) satellite photos show a plume of vog (volcanic smog) trailing ~ 60 km downwind.

Near mid-day on 20 January seismicity dropped abruptly to near background levels, whereas microphone noise became fairly continuous, indicating that the explosions had ceased but that degassing may have been continuing. The apparent cessation of Strombolian activity lasted until late 22 January, when explosions resumed. The eruption peaked about 0700 on 23 January, at which time a SIGMET (significant meteorological forecast) was issued by the FSS (Flight Service Station) Honolulu, based on pilot reports of ash up to 3-4.6 m. The explosions then decreased somewhat but were still frequent and strong through 24 January, based on the seismicity.

The EMO placed Anatahan Island off limits until further notice and concluded that, although the volcano was not currently dangerous to most aircraft within the CNMI airspace, conditions may change rapidly. Aircraft should pass upwind of, or beyond 30 km downwind from, the island, and exercise due caution within 30 km of Anatahan.

Synopsis of recent eruptions. Anatahan had no historical eruptions prior to 2003. On 10 May of that year, after several hours of increasing seismicity, a phreatomagmatic eruption sent ash to over 10 km and deposited about 10 million cubic meters of material over the island and sea. A very small craggy dome extruded during late May and was destroyed during explosions on 14 June, after which the eruption essentially ceased. A second eruption began about 9 April 2004, after more than a week of increasing seismicity. The eruption consisted of passive extrusion during mid-April, then increased to Strombolian explosions every minute or two on 24 April. The Strombolian explosions continued through mid-July, often sending a thin plume of gas and ash upwards a few hundreds of meters and 100 km downwind. Activity decreased substantially on 26 July, though visitors to the island three months later could still see very small amounts of steam and ash rising 30-40 m above the crater rim and could smell SO2 near the crater.

As discussed in our previous report (BGVN 29:12), Anatahan's third historical eruption began on 5 January 2005. Ongoing eruptions continued through at least 18 February 2005.

Anatahan lies in the Commonwealth of the Northern Mariana Islands (CNMI) ~ 120 km N of Saipan and ~ 320 km N of Guam. The first historical eruption of Anatahan began 10 May 2003 (BGVN 28:04 and 28:05); after several hours of increasing seismicity, a phreatomagmatic eruption sent ash to over ~ 9 km (~ 30,000 feet) and deposited about 10 million cubic meters of material over the island and sea. A small craggy dome extruded in late May and was destroyed during explosions on 13 and 14 June, after which the eruption ceased. The second historical eruption began about 9 April 2004 after a week or so of increasing seismicity (BGVN 29:04). That eruption primarily comprised Strombolian explosions every minute or so and occasionally sent ash up to a few thousand feet. The eruption ended 26 July 2004.

Charles Holliday (US Airforce Weather Agency, AFWA) contributed a series of remotely sensed images showing plumes in February (table 3, figure 13). The plume in the 4 February Terra image (figure 13, top) contains a brownish tinge suggesting considerable ash. The Anatahan region was on the western edge of the Terra pass. The image contains an artifact reminiscent of Venetian blinds (commonly called the bow-tie effect), which arose due to pixel replication in the mapping/processing algorithm filling in for missing data on the edge of the scan.

Table 3. A list of some of the satellite images recording Anatahan plumes during 3-15 February 2005. Those shown with an asterisk appear in the next figure. Date and time are both UTC; for example, 04 Feb 2005 at 0105 is the date and time in UTC, in this example equivalent to 04 Feb 2005 at 1105 local time. Names affiliated with satellites are as follows: DMSP (Defense Meteorological Satellite Program), NOAA (National Oceanic and Atmospheric Administration), NASA (National Aeronautics and Space Administration). Courtesy of Charles Holliday, U.S. Air Force Weather Agency.

Randy White of the U.S. Geological Survey noted that the energy release from seismic stations monitoring Anatahan dropped to near zero on 13 February 2005, yet a monitoring microphone continued to indicate considerable acoustic-energy release. Corresponding to this, a MODIS image clearly showed ash still being emitted early on 14 February (see table 3). In other words, the seismicity failed to accurately portray the eruption's vigor.

Reporters Katie Worth and Natalie Quinata wrote in the 5 February 2005 issue of the Pacific Daily News that many students in school on Guam had been sent home after experiencing dizziness or nausea because of the foul-smelling 'vog' or volcanic smog hovering over the island from the eruption. Guam lies ~ 320 km S of Anatahan.

John Ravelo wrote a news article for the Saipan Tribune published on 15 February with the title "Anatahan ash cloud continues to hinder flights." Ravelo said that, "An aircraft coming from Manila to Saipan experienced zero visibility before landing at the Francisco C. Ada-Saipan International Airport yesterday morning, prompting the carrier's pilots to fly around the island and search for a clearer approach to the runway. The passenger aircraft landed safely at the airport, but the hazy condition delayed its arrival."

At about 0909 on 14 February, the Washington Volcanic Ash Advisory Center (VAAC) reported that a plume of ash extended SW of the volcano at an altitude of 4.3 km. The plume was 18-28 km wide. Later in the afternoon, the Washington VAAC reported an ash plume below an altitude of 2.7 km that extended SW of the volcano for about 460 km. The VAAC also forecasted that the plume would shift to a more westerly direction within the next 12 hours.

According to Ravelo's 15 February Saipan Tribune article, the CNMI Emergency Management Office and the U.S. Geological Survey "said in a joint report that the magnitude of the volcanic eruption declined during the past few days." During the eruption's peak on 26 January and 1 February 2005, however, the article stated that both agencies noted that the volcano sent ash to about 4.6-6.1 km altitude.

A message from Holliday filed at 0100 UTC on 17 February 2005 included a series of remarks, mainly from unnamed scientists on the scene in the field. As background prior to presenting those remarks, we note that the term 'RSAM' (real-time seismic amplitude) signifies estimates the average amplitude of ground shaking. RSAM values increase with increases in tremor amplitude or the rate of occurrence and size of earthquakes. The RSAM estimates the seismicity during intervals when many earthquakes might occur, times when rapid earthquake-magnitude assessments might become impractical. The remarks follow.

"Over the past 24 hours, the eruptive activity at Anatahan apparently continued to decline, with RSAM levels at the seismic station ANAT now only marginally above the levels recorded just before the 5 January eruption began. Microphone amplitudes have also dropped to similar levels.

"The 2003 crater floor is now essentially entirely covered by fresh lava [with] a diameter of about one kilometer. The current eruption peaked during the period between 26 January and 2 February [2005], during which the volcano sent ash as high as 15,000 to 20,000 feet a.s.l. [~ 5,000 to ~ 6,000 m] . . .. In the days following, ash blew as far as 100 nautical miles [185 km] and vog blew nearly 600 [nautical] miles [~ 1,100 km] downwind.

"The third historical eruption of Anatahan began on 5 January, after three days of precursory seismicity. On 6 January frequent strombolian explosion signals began and by the next day ash was rising to 10,000 feet [~ 3 km] and blowing 40 nautical miles [72 km] downwind. Bombs a meter in diameter were being thrown hundreds of feet in the air [ 1 foot = 0.305 m]. By January 20 explosions were occurring every 3 to 10 seconds and fresh ejecta and small lava flows had filled the innermost crater to nearly the level of the pre-2003 East Crater floor.

"The Emergency Management Office, Office of the Governor, CNMI, has placed Anatahan Island off limits until further notice and concludes that, although the volcano is not currently dangerous to most aircraft within the CNMI airspace, conditions may change rapidly, and aircraft should pass upwind of Anatahan or beyond 30 km downwind from the island and exercise due caution within 30 km of Anatahan."

Anatahan's third historical eruption began on 5 January 2005, and is described in BGVN 29:12. Further details and satellite images were presented in BGVN 30:02, which covered events until mid-February 2005. A 5-6 April 2005 eruption cloud rose to at least 15 km altitude, which was the highest yet seen at the volcano.

Anatahan erupted almost continuously after 5 January 2005, when it started its third eruption in recorded history. An image collected by the Ozone Monitoring Instrument on NASA's Aura satellite shows atmospheric sulfur dioxide (SO2) concentrations between 31 January and 4 February 2005 (figure 14). A long SO2 plume extends NE and SW of Anatahan, and the edge of the plume covers Guam (the southernmost island) and the other Mariana Islands immediately to Anatahan's N and S.

Figure 14. Anatahan's atmospheric SO2 as imaged by Aura's OMI instrument on 31 January 2005. The OMI (Ozone Monitoring Instrument) measures SO2 in Dobson Units. Dobson Units, which derive from spectroscopic measurement techniques, can be thought of as the mass of molecules per unit area of Earth's atmospheric column. One Dobson Unit equals 0.0285 grams of SO2 per square meter. The Ozone Monitoring Instrument (OMI) that created this image tracks global ozone change and monitors aerosols like sulfates in the atmosphere. It was added to the Aura satellite as part of a collaboration between the Netherlands' Agency for Aerospace Programs and the Finnish Meteorological Institute. NASA describes the Eos system Aura as "A mission dedicated to the health of the Earth's atmosphere." NASA image courtesy Simon Carn (Joint Center for Earth Systems Technology (JCET), University of Maryland Baltimore County (UMBC)).

Volcanogenic SO2 combines with water to create a sulfuric acid haze. Called "vog," this haze can cause illness and make breathing difficult. Volcanic haze grew so thick during the first week of February that the National Weather Service issued a volcanic haze advisory for Guam, where several illnesses were reported.

After mid-February 2005, eruptive activity at Anatahan steadily declined to less than 5% of the peak level attained since the eruption started on 5 January. Ash eruptions continued, and the 2003 crater floor was almost entirely covered by fresh lava out to a diameter of ~ 1 km. A MODIS image taken at 0115 on 18 February showed a plume of steam and vog extending about ~ 170 km SW of Anatahan. Seismic and acoustic records during the last week of February 2005 showed very low levels of activity. Seismic amplitudes during 23-28 February were similar to those recorded prior to the 5 January eruption. NASA MODIS (Moderate Resolution Imaging Spectroradiometer) imagery taken on 28 February showed a faint plume of vog and steam trending W of Anatahan.

During the first two weeks of March 2005 volcanic and seismic activity increased relative to the previous weeks. During 14-17 March, seismicity increased and steam rose a few hundred meters above the volcano. The inner E crater had been nearly filled with lava flows and lapilli since early January.

A small eruption began on 18 March at 1544 according to seismic data. On 19 March the Washington VAAC issued an advisory that an ash plume was visible on satellite imagery below 4 km altitude. Small explosions that began late on 20 March lasted for 14 hours. No emissions were visible on satellite imagery, but others were, later in March and April.

A strong outburst apparently began on 21 March, a day when seismicity increased significantly. Seismic amplitudes peaked on the 25th and faded out on the 26th. Near the peak on the 25th, the U.S. Air Force Weather Agency (AFWA) detected a hot spot on the island on satellite imagery and reported an ash plume briefly reaching ~ 5.8 km altitude. The plume height soon dropped to below 3 km altitude, and by near midday on the 27th the plume had changed from ash and steam to steam and vog. On the 27th the plume extended ~ 240 km SW.

On 5 April at about 2200 seismic signals began to increase slowly, and the Washington VAAC began to see increased ash on satellite imagery. On 6 April 2005 around 0300 an explosive eruption began and produced an ash plume to an initial height of ~ 15.2 km altitude, the highest in recorded history from the volcano. Seismicity peaked at the same time.

Chuck Sayon, the superintendent of American Memorial Park noted, "On Saipan at around 10 AM the skies darkened and light ash started falling . . . park operation[s] have been restricted to indoor activities due to irritation to eyes and breathing as ash starts to lightly coat the area. Schools are closed as well as the airport until further notice . . .."

On 6 April during 0400 to 0900 the seismicity at Anatahan decreased to near background. The seismicity surged for about 1 hour, with amplitudes about one-half those reached during the earlier eruption, and subsequently dropped again to near background. Prior to the 6 April eruption, during 31 March to 4 April the amplitudes of harmonic tremor varied, reaching a 2-month high on the 3rd. Small explosions occurred every one minute to several minutes, probably associated with cinder-cone formation. Steam-and-ash plumes drifted ~ 200 km, and vog drifted ~ 400 km at altitudes below ~ 2.4- 4.6 km.

The U.S. Geological Survey (USGS) (in conjunction with the Commonwealth of the Northern Mariana Islands) stated that the "eruption of 6 April 2005 was the largest historical eruption of Anatahan and expelled roughly 50 million cubic meters of ash. The eruption column and the amplitude of harmonic tremor both grew slowly over about 5 hours and both peaked about 0300 on 6 April local time . . .. The peak of the eruption lasted about one hour and then the activity declined rapidly over the following hour."

The 6 April 2005 eruption's plume was captured on satellite images. The image showed a plume that was tan or brown in color and clearly ash laden (figure 15).

Figure 15. A major eruption from Anatahan on 6 April 2005 sent an ash plume to ~ 15 km. The eruption was considered the largest since Anatahan's first recorded eruption on 10 May 2003. This Moderate Resolution Imaging Spectroradiometer (MODIS) image was acquired by NASA's Terra satellite at 0035 UTC, about 8 hours after the eruption began. By this time, the ash plume had spread S to entirely cover Saipan and Tinian, islands immediately to the S. Courtesy of the MODIS Rapid Response Image Gallery, sponsored in part by NASA.

Figure 16 shows SO2 concentrations in the atmosphere on 7 April 2005, over 30 hours after the large 6 April eruption. SO2 emissions from the eruption were measured by the Ozone Monitoring Instrument (OMI) on NASA's EOS/Aura satellite. OMI detects the total column amount of SO2 between the sensor and the Earth's surface and maps this quantity as it orbits the planet. A new perspective on the vertical distribution of the SO2 is revealed by combining the OMI data with coincident measurements made by the Microwave Limb Sounder (MLS), also part of the Aura mission.

Figure 16. Anatahan's 5-6 April 2005 eruption injected significant SO2 high into the atmosphere. This OMI image depicts the concentrations found over 30 hours after the eruption, a time when the SO2 formed two separate zones at distance from the source. Analysis suggests that the westerly zone of SO2 was probably in the lower troposphere and the eastern zone was probably in the upper troposphere or above. Courtesy of Simon Carn.

The MLS data crisscross the OMI image and clearly show that some, but not all, of the SO2 measured by OMI to the volcano's E was in the upper troposphere or above. At these altitudes, SO2—and the sulfate aerosols that form from it—can stay in the atmosphere and affect the climate for a longer period of time. A weaker SO2 signal was also measured in the same region during the nighttime MLS overpass, which crosses the image from upper right to lower left. The daytime data, running from upper left to lower right, coincide with the OMI measurements. The MLS data west of Anatahan show no significant SO2 signal, indicating that the SO2 measured by OMI in this region was in the lower troposphere.

MLS measures thermal emissions from the Earth's limb, so unlike the OMI sensor it also collects data at night. It is designed to measure vertical profiles of atmospheric gases that are important for studying the Earth's ozone layer, climate, and air quality, such as SO2. These images, derived from preliminary, unvalidated OMI and MLS data, show MLS SO2 columns (filled circles) measured every 165 km along the Aura orbit, plotted over the OMI SO2 map. The MLS SO2 columns shown here are derived from profile measurements made from the upper troposphere into the stratosphere (~ 215-0 hPa (hectoPascal, 102 Pa) or ~ 12 km altitude and above), and the circles do not represent the actual size of the MLS footprint, which is roughly 165 x 6 km.

Anatahan's morphological changes were highlighted in before (pre-eruption) versus after (post-eruption) images (figure 17). Seismicity decreased at Anatahan after 6 April and during 7-11 April was at very low levels, near background. On 11 April, a steam-and-ash plume rose ~ 2.7 km altitude and drifted ~ 280 km WSW.

Figure 17. Two satellite images of Anatahan cropped and oriented for direct comparison, one from 20 January 2002 (pre-eruption), the other from 27 April 2005 ("post-eruption" in the sense that it was taken after the May 2003 eruption began). The NASA's Earth Observatory website described the pre-eruption image (bottom) as follows: "In 2002, when the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) captured the lower image, the island was made up of two volcanoes whose conjoined summit calderas formed an elliptical valley at the island's center. Aside from occasional tremors, the island was quiet and no eruption had ever been recorded." Green plants, shown on the false-color (infrared-enhanced) image in red, covered the island and filled the caldera at its center. The same website described the post-eruption image (top) as showing that the volcano was still emitting steam and ash on 27 April 2005. The island's center was completely devoid of plants, covered instead by gray volcanic material. Ash appears to have blanketed the western fringe of the island, where a layer of gray covers the underlying vegetation. The light cloud, ash, and steam that cover the island make it difficult to see changes to the caldera, but it appears that the eruptions may have destroyed its southern wall. On higher resolution images, it also appears that volcanic material may have flowed into the Pacific Ocean on the island's S side. Courtesy of NASA's Earth Observatory.

Occasional data from Anatahan revealed that seismicity appeared to increase during 24-25 April. During 20-25 April, a continuous thin plume of ash-and-steam rose to less than ~ 3 km altitude and drifted more than 185 km from the volcano. Harmonic tremor dropped dramatically on 1 May after being at high levels for several days. During 27 April to 1 May, the main ash-and-steam plume rose to ~ 3 km altitude According to a news article, the volcanic plume from Anatahan reached Philippine airspace on 4 May.

On 5 May an extensive ash-and-steam plume to 4.5 km altitude was visible in all directions. Ash extended 770 km N, 130 km S (to northern Saipan), and 110 km W. Vog extended in a broad swath from 3,000 km W, over the Philippines, to 1,000 km N of Anatahan. By 9 May harmonic tremor amplitude had decreased to near-background levels, with a corresponding drop in eruptive activity. As of 10 May AFWA was reporting ash to about 3 km altitude extending 400 km W and an area of vog less than half that noted on 5 May.

On 11 May AFWA reported thick ash rising to 4.2 km altitude and moving WNW. The thick ash extended in a triangular shape from the summit 444 km to the WSW through 510 km to the NW. A layer of thin ash at 3 km altitude extended another 1,000 km beyond the thick ash. A broad swath of vog extended over 2,200 km W nearly to the Philippines and over 1,400 km NNW of Anatahan. Although the ash plume diminished over the next few days and was not as thick, it remained significant, rising to 2.4 km and extending 370 km WNW on the 13th. Scientific personnel from the Emergency Management Office and the USGS working the next day at a spot 2-3 km W of the active vent heard a continuous roaring sound. They also saw ash and steam rising by pure convection, not explosively, to 3 km altitude.

Anatahan's third historical eruption began on 5 January 2005 (BGVN 29:12 and 30:02). On 5-6 April 2005, an eruption cloud rose to 15.2 km altitude, the highest yet seen at the volcano (BGVN 30:04). That eruption, estimated to have expelled 50 million cubic meters of ash, caused the temporary closure of Anderson Air Force Base on Guam. An eruption that began on 5 May and produced an extensive ash and steam plume was briefly described in BGVN 30:04, but further details follow. Plumes were frequently visible in satellite imagery; a summary of satellite observations is presented for 16 June-20 July 2005 (table 4).

Observations during early May 2005. Activity surged to a moderately high level on 5 May, when an extensive ash-and-steam plume to 4.5 km altitude was visible in all directions. Ash extended 770 km N, 130 km S to northern Saipan, and 110 km W. Vog extended in a broad swath from 3,000 km W, over the Philippine Islands, to 1,000 km N of Anatahan. By 9 May harmonic tremor amplitude had decreased to near background levels, with a corresponding drop in eruptive activity. As of 10 May the Air Force Weather Agency (AFWA) reported ash rising to about 3 km altitude and extending 400 km W, with an area of vog less than half that noted on 5 May.

Anatahan began erupting suddenly from its E crater at about 1700 on 10 May. Within hours of the eruption's onset, a towering column of volcanic ash and gas rose to more than 10 km altitude, and the prevailing wind blew the ash W. An immediate concern was the potential for the tiny abrasive ash fragments to damage aircraft passing nearby and downwind from the volcano. The Washington Volcanic Ash Advisory Center issued an advisory that volcanic ash was present at 11 km altitude moving S at 65 km/hour and at 4.6 km altitude moving W at 20-30 km/hour.

The single seismic station on the island maintained by the Emergency Management Office of the Commonwealth of the Northern Mariana Islands (EMO/CNMI) was not working at the time, but a broadband seismic instrument installed 6.5 km W of Anatahan's crater on 6 May by scientists from Washington University in St. Louis recorded significant earthquake activity in the hours before the eruption began; the instrument was one of many installed to conduct a seismic experiment along the Mariana Trench. A preliminary review of the data shows there was a rapid increase in the number of small-magnitude earthquakes (probably less than M 2) to more than 100 per hour beneath the volcano within a few hours of the eruption onset.

A smaller but nearly continuous eruption column rose from the E crater of Anatahan for several days following 10 May. The resulting eruption clouds were generally below about 6 km altitude. On 11 May AFWA reported thick ash rising to 4.2 km altitude and moving WNW. The ash extended in a triangular shape from the summit 444 km to the WSW through 510 km to the NW. A layer of diffuse ash at 3 km altitude extended beyond the dense ash for another 1,000 km. A broad swath of vog extended over 2,200 km W nearly to the Philippines and over 1,400 km NNW of Anatahan. Although the ash plume diminished over the next few days, it remained significant, rising to 2.4 km altitude and extending 370 km WNW on 13 May. Personnel from EMO/CNMI and the U.S. Geological Survey (USGS) who were repairing and installing equipment on 14 May reported hearing a continuous roaring sound from 2-3 km W of the active vent. They also saw ash and steam rising by pure convection, not explosively, to 3 km altitude.

Observations during later May and June 2005. Following nearly continuous eruption from January through April 2005, on 23-24 May typhoon Chan-hom shifted the prevailing E winds to the S, blowing the eruption column toward Saipan and Guam. Light ashfall resulted in flight cancellations at the Saipan and Guam international airports. Residents of Saipan reported a rotten-egg smell associated with the ashfall. The ongoing explosive activity excavated a deep crater within Anatahan's E crater. Scientists estimated the inner crater was nearly at sea level by about 20 May; before the eruption, the floor of E crater was 68 m above sea level.

The spiny surface of a lava flow was first observed in the inner crater on 4 June. The flow appeared to form a mound-shaped lava dome, but its volume is unknown. New fault scarps and slump features were seen within the E crater, as well as additional faulting W of the E crater. A gradual increase in the number of long-period (LP) earthquakes and tremor began at Anatahan on 5 June. Both LP and tremor events peaked during 2230-0030 on 6 June. During the peak in activity, more than 350 LP events occurred. Tremor amplitudes briefly reached a new high for the current eruptive activity, and an ash column reached ~ 7.9 km altitude. On 6 June, tremor amplitudes returned to low levels. During the rest of the week of 1-7 June, ash plumes reached a maximum altitude of 4.3 km. On 5 June the EMO/CNMI seismic station was repaired and ash samples were collected from the site. Through 12 June, the seismic records showed only continuous ground shaking to varying degrees. The most intense periods of tremor lasted 3 to 10 hours and occurred about every 24-36 hours.

On 12 June, three LP earthquakes were recorded, the largest about M 2. Other earthquakes followed in the late afternoon and early evening of 13 June. During 17-26 June 2005, seismicity was at the highest level since the eruption on 6 April, with real-time seismic amplitude (RSAM) values at ANA2 consistently near 625.

Since 18 May, Anatahan has sent ash and steam continuously to 2.4 km altitude or higher, with seven eruptive pulses to 7.6 km altitude or higher. On 11 June, a 10 minute-long eruptive pulse sent ash and steam to 14 km altitude. On June 19, a 2.6 minute-long eruptive pulse sent a cloud of steam and ash to 15.2 km altitude; the cloud moved E and dissipated after about 7 hours. On 6 July, very high levels of tremor for about 30 minutes accompanied an eruptive pulse to 12.2 km altitude.

On 11 June beginning at 1622 three explosions produced a dense ash cloud that rose to an altitude of ~ 13.7 km. On 12 June, seismicity was at moderately high levels, with periods of strong tremor and frequent small LP earthquakes. Satellite imagery showed an ash cloud at an altitude of ~ 3 km.

Two strong explosions on 14 June removed much of the small new dome in the inner crater. Just before noon on 14 June, earthquakes began to occur at intervals of 1-2 minutes. For the next two days, episodes of intense tremor and earthquakes lasting about 1.5 hours occurred about every 12 hours, accompanying strong ash emissions from the E crater, with eruption columns higher than 2 km altitude. Quiet intervals in which the eruption column contained little ash were accompanied by continuous weak tremor.

On 19 June at 1525 a brief eruption produced a steam-and-ash cloud that reached an altitude of ~ 15.2 km (figure 18). Guam Meteorological Weather Office radar showed that the cloud drifted E. No seismic signal was clearly associated with the eruption. Two days before the eruption, the amplitude of continuous tremor was relatively high. During the days before and after the eruption, ash reached 3-4.6 km and drifted W.

Figure 18. NOAA satellite image of the ash plume at FL500 (15.2 km) from the Anahatan eruption of 19 June 2005. This was one of the highest plumes ever recorded from the volcano. Its position SE of Anahatan is unusual; the usual direction of the ash and other emissions is W. Courtesy of US Air Force Weather Agency.

During 22-27 June AFWA observed, on satellite imagery, a moderately dense cloud of ash and steam that rose to a maximum altitude of ~ 3 km, and drifted W. Additional thin ash and vog were visible to the W and NNW of the island. On 26 June AFWA identified, on satellite imagery, a dense cloud of ash and steam rising to ~ 3.7 km, moving towards the W, and vog to the W, N and NE of the island (figure 19). No particular seismic signal was associated with the eruptions. By 28 June the seismicity level dropped by about 80% from the continuously high levels of the last week.

Figure 19. GOES-9 image of 30 June 2005 showing the extent of the atmospheric injection of Anahatan ash and gas. The emission reached the island of Kyushu. Courtesy of US Air Force Weather Agency.

On 3 July at 1646 an eruption produced a SSE-drifting plume to an altitude of ~ 12.2 km, according to Guam Meteorological Office radar. Vog briefly drifted S over the islands of Saipan and Tinian. During 29 June to 5 July, steam-and-ash emissions continued to rise to low altitudes. During 6-11 July, eruptive activity continued, with steam-and-ash plumes rising to a maximum altitude of 6.1 km. On 6 July beginning at 1730 tremor at the volcano increased, and an eruption produced an ash plume to an altitude of ~ 12.2 km. During 8-11 July, a thin layer of vog extended over much of the Philippine Sea (figure 20).

Figure 20. GOES-9 image of 8 July 2005 showing the ash plume and vog from Anahatan extending 2,450 km W almost to the Philippines and Taiwan. Courtesy of US Air Force Weather Agency.

As of 1 August 2005, Anatahan was presumed to be in a state of constant eruption. For the first half of 1 August, volcanic tremor levels as recorded at Anatahan's E seismic station (ANA2) were between 40 and 60 % of the peak levels observed during 17-26 June. At 0800, the National Weather Service at Tiyan, Guam, issued a volcanic ash advisory for Saipan and Tinian. A strong sulfur odor from the emitted volcanic gases was reported by numerous residents, and ash was observed on the tips of aircraft at Saipan International Airport. Traces of ash were also apparent on solar panels powering equipment run by the EMO/CNMI on Saipan. According to the Air Force Weather Agency, continued cloud cover caused by a tropical storm inhibited ash detection on METSAT imagery. As of 1252 on 1 August, the ash plume was presumed to be at an altitude of 4.6 km, moving toward the S at 18-27 km/hour.

Anatahan has erupted almost continuously since 5 January 2005, when it started a new episode of vigorous discharges. A summary of satellite images during 16 June-20 July 2005 (BGVN 30:07) showed that it was one of the most conspicuous eruptions on the planet in 2005.

Eruptions somewhat abruptly ceased on about 3 September. Significant discharges remained absent through as late as 29 September. Activity described below through early September is based on an array of material from numerous sources, including the US Geological Survey (USGS), the Washington VAAC, the U.S. Air Force Weather Agency (AFWA), the press, and the Emergency Management Office of the Commonwealth of the Northern Mariana Islands (EMO-CNMI).

Following the general discussions of activity, a report is included contributed by Setsuya Nakada from the Earthquake Research Institute (ERI), University of Tokyo, whose team of scientists made close-range observations of a distinctive eruptive phase called an ash-cloud surge from a helicopter on 24 August. On that day they documented the surge as a robust sub-horizontal plume slowly traveling over, and in contact with, the ocean surface.

Activity during August 2005. Throughout August eruptive activity continued, with plumes rising several thousand meters above the volcano. On 1 August and during 3-9 August the National Weather Service at Tiyan, Guam, issued numerous reports for the islands of Saipan and Tinian.

On 1 August a strong sulfur odor was reported by numerous residents, and ash was observed on aircraft at Saipan International Airport. According to a news article, flights leaving the airport were delayed.

A 4 August article published in the Saipan Tribune by John Ravelo was entitled "Engine trouble forces aircraft's emergency landing." What follows comes from the opening paragraphs of that article, describing events attributed to 3 August.

"An aircraft suffered engine trouble in mid-air early last night shortly after taking off from the Saipan International Airport, prompting it to return to the tarmac for emergency landing. The Ports Police said no one was injured in the incident. This happened as Saipan, Tinian and Rota remained under volcanic haze from Anatahan until last night . . ..

"The aircraft reportedly left the Saipan airport at approximately 6 pm. Minutes later, at about 6:15 pm, Ports Police on-duty airport supervisor Sgt. Greg Arriola said his office received a call that the aircraft was coming back due to 'problems with its left engine.'

"'The aircraft landed safely. Everybody was safe,' Arriola said. He refused to elaborate and name the aircraft, saying, 'we're still checking [on] the matter.'

"Arriola did not disclose the number of passengers aboard the distressed aircraft and where the plane was supposedly bound. The haze over Saipan has resulted in flight interruptions since Monday [1 August], temporarily stranding hundreds of passengers.

"According to the U.S. Geological Survey, volcanic ash threatens jets ... as it forms deposit in engines, restricts airflow, and clogs fuel nozzles. Minute particles of volcanic ash also contaminate aircraft's ventilation, lubrication, hydraulic and electronic systems. They cause erosion and pitting of leading edges of windshields and landing lights, as well as erosion of compressor blades."

The USGS and EMO noted that a seismic station on Sarigan, Anatahan's neighboring island 6.5 km to the W, recorded more than 40 earthquakes on 9 August, three of which had magnitudes of around 4. The seismic swarm began at around 0152 and occurred over the next eight hours. At around 0539, an M 4.2 earthquake occurred, and the National Earthquake Information Center traced the event to ~ 65 km NW of Anatahan.

Tremor and long-period earthquakes were recorded through 8 August. Later, an ash plume was detected on satellite imagery by the AFWA and Washington VAAC; it was at 5-5.5 km altitude extending approximately 220-400 km NW from the summit on 9 August. A Washington VAAC report that day was the 650th Anatahan report they had issued in 2005.

During the remainder of August 2005, eruptive activity continued and ash plumes rose to 6-8 km altitude. Volcanic tremor levels ranged between 20 and 65 percent of peak levels, and long-period earthquakes occurred sporadically. But, after around 0205 on 27 August, the seismic station went off-line. During 1-3 September activity continued, with ash plumes rising to a maximum of ~ 3 km altitude.

Eruptions halt on 3 September 2005. The USGS reported that based on remote-sensing data Anatahan appeared to have stopped erupting on 3 September (UTC), and initial data documenting that circumstance represented observations between 0901 and 2308 UTC. The earlier time corresponded to when AFWA last noted visible ash on a GOES 9 image. Ash could not be detected in satellite imagery through 1825 UTC due to cloud cover. A pilot report at 2308 UTC on 3 September indicated "no activity was occurring at the volcano." The Washington VAAC reported that no ash was detected in satellite imagery through 0025 UTC on 4 September under mostly clear skies. MODIS imagery at 0040 and 0345 UTC on 4 September also show no discernible ash being erupted under clear skies.

This was the first time that the USGS and the EMO reported an absence of volcanism on Anatahan since ash-bearing discharges started in early January 2005. Since then, tremor levels have been fluctuating, with occasional Strombolian explosions.

During an overflight the week of 7 September, USGS and EMO personnel did not see any ash emissions, only low-level steam-and-gas emissions. They noted that the crater floor was covered by sediment-laden water. In East Crater they saw an active geothermal system, consisting of mud pots, mini-geysers, and steam jetting from the crater walls.

Although volcanic seismicity was at low levels through at least 16 September, according to the World Data Center for Seismology (Denver, Colorado) a M 4.4 earthquake struck the Saipan region of the Northern Mariana Islands on 9 September 2005. It occurred at about 1301 local time, with the epicenter 80 km SSW of Anatahan.

Eruption observations, 24 and 26 August 2005. The following describes a 24 August helicopter visit to the vicinity of Anatahan, which included witnessing and documenting eruptive phenomena, but not landing. Photos were also taken on 26 August while passing well to windward of the island on a commercial airliner. The report was submitted by Setsuya Nakada, who was accompanied from Japan by colleagues Takeshi Matsushima (Institute of Seismology and Volcanology, Kyushu University), and Mitsuhiro Yoshimoto (Volcano Research Center, Earthquake Reserch Institute, University of Tokyo).

They had hoped to recover GPS and tiltmeter data from stations on Anatahan, and to find, exhume, inspect, and repair any ash-covered instruments. These instrumental data span the important period starting from last year, an interval that could shed light on the behavior of the volcano and the magma system during the eruption. Geological inspection and petrological sampling were also planned.

Anatahan's seismicity changed from continuous, strong signals to undergoing intermittent pulsations around the morning of 23 August, and a large (M 4.8?) LP earthquake occurred at 2045 on 23 August. Judging from this sudden seismological change and the LP event, the USGS scientists monitoring the seismicity (Andy Lockhart, Randy White, and others) suggested suspending landing on the island for at least a few weeks.

The team decided to fly over the island without landing to assess the state of burial of their geodetic observation site. They spent about an hour in the air there (about 1000 to 1100 on the 24th) viewing and photographing the scene. Besides the pilot, the helicopter carried Nakada, Matsushima, Yoshimoto, and Juan Camacho (EMO-CMI).

During the 24 August visit, dense ash clouds issued very vigorously from the active E crater. The cloud and hung over the summit calderas, their SW rims, and swept out over the sea to the SW and W of the island.

The photographs and the impressions of Camacho and his pilot from visits in July and May 2005 suggested that the activity level was higher on 24 August. The flight disclosed an island completely covered with thick layers of both wet (dark and probably very fine) ash deposits, and fresh dry ones on the island's S slopes. The dry ones lay under the ash cloud. Green areas were restricted to spots on the outer slopes. Many gullies had begun to develop on the surface of the thick ash deposit.

The observers saw a dark eruption cloud (densely ash-laden) vigorously blasting out of the active crater. A heavy ash cloud hung over the island (figures 21 and 22). The eruption cloud rose to ~ 800 m directly above the crater, and it increased up to ~ 2,000 m over the W part of the island, where it became lighter in color. Darker, vigorous emissions also came from the east crater's W side or NW side, and less frequently from its E side. Though this may have reflected the complex circulation of air within the east crater, another possibility was that the active crater had widened recently, especially to the E. Two ash emission points may have developed inside the large active crater.

Figure 21. View of Anatahan and plumes looking NW, taken from a helicopter around 1000 on 24 August 2005. The windward area is mantled with a light-colored, low hanging plume (far right). A large dark plume emerges from the vent (east crater); as it hugged the sea surface it advanced roughly horizontally and comparatively slowly A spike of light-colored cloud rises above the darker plume, over an area over the sea but not far from the island. Courtesy of T. Matsushima.

Figure 22. View of Anatahan looking from the S taken during the helicopter inspection around 1000 on 24 August. In addition to the eruptive clouds, this photo includes the eruptive vent area and some portions of the tephra covered island. Courtesy of T. Matsushima.

Seismic amplitudes during the flight were weaker than recorded the afternoon of 23 August. Seismic signals consisted of intermittent pulses with duration intervals from 5 to 20 minutes. Such signals could presumably have corresponded with a series of Strombolian explosions, but on the flight these were not seen. No projectiles were observed—even near the base of the eruption cloud—although the vent was obscured by a profusion of drifting clouds (figures 1 and 2). Abundant ash-laden clouds passed vigorously and continuously from the active crater, escaping in cycles of five's to ten's of minutes in duration, intervals seemingly similar to the seismicity during the flight.

Abundant ash fell from the dark ash cloud that drifted to the SW of the crater. Around 1000 a ring of ash-cloud surge expanded on the crater's southern rim. It advanced comparatively slowly, traveling SW (figures 21 to 24). Along the sea surface, many small lobes of ash cloud developed, moving slowly. These were reminiscent of lobes seen in surges observed at the Tar River Valley delta during the Soufrière Hills eruption. These eruptive scenes also appeared very similar to those observed on 29 August 2000 at Miyake-jima (Nakada and others, 2005a), where a low-temperature ash-cloud surge moved slowly from the summit crater. In the case of the ash-cloud surge seen at Anatahan, it may be that the passage across sea water had a profound influence, triggering behavior more closely phreatomagmatic than purely magmatic in character. The ash-cloud surge took place mainly as the observers approached the island. The surge was thought to correlate to an interval of elevated seismicity.

Figure 23. A close-up photograph documenting ash-cloud complexities from the eruption at Anatahan at about 1000 on 24 August. Looking N, the photo focused on a part of the dark ash cloud above the ocean. The plume is both dropping ash and billowing upwards. Along the cloud base and adjacent the ocean surface grew a light-colored fringe of expanding clouds. Courtesy of S. Nakada.

Figure 24. A N-looking photo of the 24 August eruption at Anatahan that is similar to the previous one, but better illustrating the rising upper portions of the dark ash cloud. Courtesy of M. Yoshimoto.

Tephra buried portions of the village ~ 7 km W of the active crater and reached 1.5 m thick. A photograph revealed that the GPS antenna, within a 50-cm-high pillar, remained distinct even though under considerable ash. The cable to the computer was also partly visible inside a collapsed hut, suggesting the prospect of still retrieving the data. The GPS end-point station ~ 1.5 km E of the crater was under a ~ 1-m-thick blanket of ash, but again the GPS antenna was seen on the edge of a small pond.

A thermal imaging camera system took an essentially simultaneous thermal (infrared) image and a visible-light photograph (figure 25). The eruption cloud was too dense to capture the temperature distribution near the floor of the crater area. Instead, the images represented only the temperature distribution of the cooler, outer portions of the clouds, where temperatures ranged from 19.5 to 27°C (figure 25).

Figure 25. Anatahan's vent area emitting copious rising clouds as recorded in both a conventional (visible wavelength) photograph and a nearly simultaneous (infrared wavelength) thermal image. The photographer was looking northward from S of the crater around 1100 on 24 August. The shots were made with a thermal imaging camera (Thermo Tracer TH9100MV, NEC San-Kei Instruments, Ltd.) that takes the thermal and visual images in rapid succession. Courtesy of M. Yoshimoto.

As Nakada and colleagues departed from the Mariana Islands, Anatahan's eruption was seen again (figure 26), this time from a commercial air flight (Northwest Airlines' flight NW0078) traveling from Saipan to Nagoya and departing at 0930 on 26 August 2005. The plume was directed SW. In addition to the very different plume morphology seen that day, the eruptive intensity was judged to have been higher than on 24 August (figure 26).

Figure 26. Three views of Anatahan looking SW as it discharged a large, buoyant ash plume on 26 August 2005. Photographs were taken en route from Saipan (CMI) to Nagoya (Japan). Courtesy of S. Nakada.

Anatahan erupted almost continuously from 5 January 2005 until 3 September 2005 when eruptions suddenly ceased (BGVN 30:07, 30:08). Observations through 16 September indicated relative quiet. Indications from later reports (discussed below) are that this lull continued through at least mid- to late-February 2006. Eruptions resumed after that, although the observations suggest chiefly or entirely gas-rich plumes. Jenifer Piatt suggested that plumes after early September 2005 and through May 2006 rose only to low altitude, perhaps 2,500 m.

This report covers the period through early June 2006 and includes both field observations as well as several satellite-based SO2 measurements, and extensive satellite images of thin plumes assessed as vog (volcanic smog; table 5). Some of those plumes extended W to SW from Anatahan and had overall atmospheric SO2 masses on the order of up to 4 kilotons (kt).

Table 5. AURA/OMI SO2 from Anatahan plumes at stated dates in 2006 (the two indicated with asterisks ("**") shown as figures). The last column displays the plume's overall estimated SO2 mass. The second and third columns indicate, respectively, the area of the sulfurous plume, and the estimated maximum SO2 concentration (in DU) and its latitude and longitude. Courtesy of Simon Carn.

During the week ending 19 September 2005, there were three periods of elevated tremor. On 13 September, technicians from the Emergency Management Office of the Commonwealth of the Northern Mariana Islands (EMO-CNMI) who were reinstalling seismic station ANA2 on Anatahan reported that the plume was gray, small, and moving to the NW. They heard no explosions and saw no craters or large ballistics in vicinity of ANA2.

CMNI-USGS reports for 3 September until at least 26 December 2005 noted an absence of erupted ash. At least as late as 27 February 2006, Anatahan lacked reported ash emissions. Also as late as the 27th, seismicity was at background levels, amounting to a few percent of the late June 2005 maximum, with occasional long-period earthquakes. On 27 February 2006, the Alert level was reduced to Normal and the Aviation Color Code to Green because of the continuing low levels of activity.

By the date of the next USGS update, on 20 March 2006, activity had increased somewhat and the Alert level was raised to Advisory and the Aviation Color Code to Yellow. A faint, thin plume of gas that was occasionally observable during January and February became continuous and slightly more dense on satellite imagery during the first three weeks of March.

Using the Ozone Monitoring Instrument (OMI) on NASA's EOS/Aura satellite, Simon Carn imaged Anatahan's plume of 15 March 2006 (figure 27). Anatahan lies at the solid triangle; the plume blew largely SW. Carn found that the atmospheric SO2 mass was 1-2 kilotons. He noted that there had been an upsurge in satellite-detected SO2 output that began in mid-February 2006. The highest concentrations of several OMI analyses (table 6 and figure 28) were measured on 16 and 23 April (3.9 and 3.5 kilotons of SO2, respectively).

Table 6. A summary of Anatahan plume data based on US AFWA satellite observations during 15 March to 31 May 2006. DMSP stands for Defense Meteorological Satellite Program. Courtesy of Charles Holliday and Jenifer E. Piatt, AFWA.

Figure 27. AURA/OMI image of SO2 from Anatahan at 0400-0420 UTC on 15 March 2006 (orbit 08852). The overall estimated SO2 mass in the 15 March plume was 1-2 kilotons. Concentration path-lengths for the atmospheric column are scaled in Dobson Units (DU). This is an example of a comparatively short plume, with greatest SO2 concentrations nearest the source, and blown somewhat more southerly than some of the later ones. Courtesy of Simon Carn.

Figure 28. AURA/OMI image of SO2 from Anatahan at 0249-0428 UTC on 12 April 2006. The overall estimated SO2 mass in the 12 April plume was 2.2 kilotons (for other parameters and comparisons, see table 6). This is an example of a comparatively elongate plume, with highest SO2 registered in areas ~1,000 km ESE of the source. Courtesy of Simon Carn.

OMI is a Dutch-Finnish imaging spectrometer that measures ozone and other atmospheric trace gases such as SO2. OMI is a nadir-viewing imaging spectrometer that covers the ultraviolet and visible spectral range (270-500 nm). Its high spatial resolution increases the chance of observing cloud-free pixels, thereby enhancing the accuracy of the data products. OMI observes a strip of the Earth's surface about 2,600-2,800 km wide in one shot. The satellite's own movement along with Earth's rotation enables OMI to scan the entire globe. A two-dimensional CCD detector records both the complete swath and the spectrum of every ground pixel in the swath. The spatial information is imaged on one dimension of the CCD detector while the spectrum is projected along the other dimension of the CCD detector. OMI detects the total column amount of SO2 between the sensor and the Earth's surface and maps this quantity as it orbits.

On 17 March around 2200 UTC, the level of seismicity nearly doubled and continued at that level for 2 hours. On the 18th around 1400 UTC, the level of seismicity again nearly doubled and continued at that level for about 8 hours before returning to the baseline level prior to 17 March. The increased seismicity consisted of small (M 0-1) long-period earthquakes occurring approximately every minute, sometimes reaching two per minute. A total of about 600 such events were detected during 17 and 18 March. Volcanic Ash Advisories were issued by the Washington VAAC; plumes appeared to contain gas and only insignificant amounts of ash.

According to the Air Force Weather Agency (AFWA), on 19 March a hot spot at Anatahan was visible on satellite imagery. Vog (volcanic smog) extended 200 km from the island (figure 29).

Figure 29. Anatahan's SW-drifting plume at 0320 UTC on 19 March as seen in a satellite image (AQUA MODIS, 500 m resolution) The US Air Force Weather Agency (AFWA) analysts interpreted this plume as vog. Courtesy of AFWA and NASA.

On 24 March around 1330, seismicity at Anatahan abruptly increased to about twice the background level. The seismicity consisted of low-amplitude tremor and small, long-period earthquakes, similar to the seismicity on 17 and 18 March. On 24 March, vog from Anatahan was visible on satellite imagery extending W, then curling N. The plume was estimated to be below 1.2 km altitude, and no ash or hot spots were visible. Anatahan remained at Alert level Advisory; Aviation Color Code Yellow (Volcanic activity has increased somewhat, but remains fairly low and is being closely monitored).

From 28 March to 4 April, seismic levels fluctuated. Seismicity again jumped up to about double the background level for a few hours on 29 and 31 March and 2 April. Anatahan continued to produce a gas-and-steam plume visible in satellite imagery. On 4 April, Saipan residents reported smog and the smell of sulphur.

On 8 April a team from EMO-CNMI visited Anatahan and found steam and gas discharging from the E crater along the SW crater wall above a discolored lake. Testing confirmed the presence of SO2 and H2S in the plume. The plume rose to an altitude of less than 2 km and drifted to the NW as brownish vog. No ash fell from the plume onto the island. Based on these results and satellite surveillance, Anatahan was inferred to be emitting steam, gas, and vog.

Three long-period earthquakes occurred on 14 and 15 April. Each was preceded by several minutes of significantly reduced seismicity. AFWA reported that a hot spot was visible on NOAA shortwave IR imagery on 17 April at 1612 UTC, and vog extended over 490 km WSW in F-13 imagery on 17 April at 2143 UTC. SO2 mass values for 23 April were the second highest in this reporting interval. On 24 April 2006 AFWA reported that hot spots were occasionally visible and that vog was nearly always visible in satellite images.

Throughout May 2006, Anatahan's E crater continued to emit vog that was visible in MODIS imagery. Seismicity levels were low throughout April and May. A few to several microearthquakes occurred each day, all with magnitudes M 1 or smaller.

Ash may have erupted in late May. Although ash was indicated on radar on 27 May, and in a pilot's report for 29 May, those events took place during intervals of such low seismicity that people watching that data felt eruptions were unlikely to have occurred then.

On the other hand, based on a pilot report, the Washington VAAC declared that an ash plume from Anatahan reached an altitude of 3 km on 29 May and drifted W. Vog issuing from the E crater was visible on satellite imagery at about 1333 on 29 May 2006, and increased prior to emission of an ash plume. A report issued from the Washington VAAC on 30 May at 0535 indicated a faint, low-level gas-and-ash plume extending from the summit. At 2120 UTC on 30 May the plume extended over 1,480 km WSW.

By 19 June continued gas and steam emissions remained visible in satellite imagery. Seismicity dropped from recent levels and occasional microearthquakes were recorded locally.

Regular plume emissions seen in satellite imagery and by aviators during March-May 2006 (BGVN 31:05) apparently ended in June, with the last reported activity being a pilot report of an ash cloud on 26 June that reached 3 km altitude. A report issued by the U.S. Geological Survey (USGS) on 7 December noted that the Alert Level was being lowered to Green and that seismic activity at Anatahan was very low during late November and early December, although diffuse steam-and-gas plumes were occasionally visible on recent satellite images or by aviators.

According to the USGS, seismometers recorded tremor starting on 24 February (UTC) that continued at high levels through 17 March. During that time, recorded tremor occasionally increased to much higher values. In addition, OMI satellite spectrometer data showed occasionally high amounts of sulfur dioxide over Anatahan. Tremor levels increased significantly starting at 1625 on 9 March (UTC) and continued for over 40 hours. As of 13 March the tremor bursts were infrequent, and some were high amplitude. In addition, a distinct gas plume was visible in Moderate Resolution Imaging Spectroradiometer (MODIS) imagery, suggesting increased emissions. On that day the Alert Level was raised to Advisory.

The MODIS flying onboard the Aqua satellite captured a view of the plume on 18 March 2007 as emissions continued. In the image, the volcanic plume headed SE, then changed direction slightly and trended towards for the islands of Saipan and Tinian. The same day MODIS acquired this image, the U.S. Air Force Weather Agency reported an odor of sulfur, which would also suggest the presence of vog (volcanic smog) on Guam, ~200 km SW of Saipan. USGS and Emergency Management Office air quality instruments on Saipan recorded a maximum 5-minute average of 959 ppb sulfur dioxide and 99 ppb hydrogen sulfide on 18 March.

As of 24 March, the USGS was reporting that tremor levels after 17 March had remained low at pre-24 February levels. The plume visible in MODIS imagery had also remained weak but distinct since 18 March. On 24 March the Alert Level was lowered to Normal, with an aviation color code of Green. No confirmed ash eruptions had occurred after 3 September 2005.

During mid 2007 and into early February 2008, Anatahan (figure 30) discharged occasional significant plumes, as restless seismicity associated with intermittent eruptions continued. Key source data for this report came from the U.S. Geological Survey (USGS), the Emergency Management Office of the Commonwealth of the Northern Mariana Islands (EMO), NASA Earth Observatory, and the Washington Volcanic Ash Advisory Center (VAAC).

Figure 30. Satellite image of Anatahan in early 2008. Bright white areas are clouds. A diffuse plume dirfting NW appears to be originating from fumaroles in the eastern crater. Courtesy of Google Earth and Digital Globe, accessed 20 February 2008.

The same day MODIS acquired this image, the U.S. Air Force Weather Agency reported an odor of sulfur on the island of Guam, ~ 200 km SW of Saipan, which also suggests the presence of vog. USGS and EMO air quality instruments on Saipan recorded a maximum 5-minute average of 959 ppb sulfur dioxide (SO2) and 99 ppb hydrogen sulfide (H2S) on 18 March. Although such plumes can cause closure of the Saipan airport and result in health risks to Saipan residents, such problems were not mentioned in reports of this incident.

Crater lake disappears. Seismicity remained restless and intermittent plumes continued to discharge from Anatahan during late 2007 and into 2008. By 31 January 2008 the crater lake had disappeared.

Distinct increases in amplitude of seismic tremors occurred on 26 and 28 November 2007. Explosions were also observed, with rates peaking on 28 November at several per minute. This increase prompted raising the alert level to Yellow/Advisory on 29 November.

On 14 December 2007 the Washington Volcanic Ash Advisory Center (VAAC) reported a steam plume visible in satellite data, but no indication of ash. There was a small surge in seismic activity recorded on 16 December that decreased to previous levels by the following day. According to the U.S. Air Force Weather Agency, a plume on 30 December consisted primarily of steam and gas, with little ash content. Seismic tremor levels increased 16 January 2008 and persisted.

On 31 January 2008, satellite data showed that the lake in the E crater, a water body whose level had been dropping since September 2007, had disappeared. According to the USGS, the tremor indicated that the volcano may have entered a new phase within its current episode of unrest, and the disappearance of the lake suggested that the magmatic heat source may have moved closer to the surface.

Ash emissions occurred at Anatahan on 3 February 2008. Satellite images showed a diffuse ash plume extending W for ~ 100 km. It was not possible to determine precisely the altitude of this ash plume from the currently available data, but it was likely less than 1,500 m. On 5 February, the USGS reported persistent elevated seismic tremor and continued detection of SO2 in satellite data. The USGS changed the Aviation Color Code to Orange and the Alert Level to Watch as a result of the ash emissions.

A satellite image (figure 31) shows the volcanic island on 6 February 2008. Dwarfed by clouds overhead, the island released a faint plume (presumably bearing little if any ash) blowing WNW. Data from the satellite-based Ozone Monitoring Instrument (OMI) showed a low-level SO2 plume extending W to SW from the volcano.

Figure 31. Anatahan released plumes of ash and steam in early February 2008, continuing a pattern of intermittent activity from the previous December. The Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA's Aqua satellite captured this image on 6 February 2008. In this image, a diffuse pale gray plume blows W from the volcanic island and over the Pacific Ocean. NASA image courtesy the MODIS Rapid Response Team at NASA GSFC.

No thermal anomalies have been measured by MODIS satellites over Anataham since June 2006. In a recent publication, Hilton and others (2007) reported on newly derived SO2 emission rates for Anatahan.

The eruption at Anatahan (figure 32) had continued through February 2008 with intermittent eruptions (BGVN 32:12). This report covers 6 February 2008 into early January 2009 but also draws on an older reference on the status of coral reefs (Starmer, 2005). No thermal alerts have been measured by MODVOLC at Anatahan since 5 June 2006.

Figure 32. Location map showing Anatahan and Sarigan. The bathymetry data are a combination of satellite bathymetry overlaid with EM300 multibeam bathymetry, which was collected on the NOAA Submarine Ring of Fire 2003 cruise aboard the R/V Thompson. Courtesy of NOAA Ocean Explorer (modified from original).

Ash plumes, SO2 emissions, and elevated seismicity continued between February and August 2008 (table 7). However, activity was consistently low after mid-August 2008. The last posted Volcanic Ash Advisories appeared on 1-3 August 2008. Throughout the reporting interval a key focus of reporting were episodes of sulfur-dioxide (SO2) emissions that drifted S to inhabited islands.

Table 7. Activity reported at Anatahan by week during 6 February 2008 to 31 January 2009. VAL is Volcanic Alert Level, ACC is Aviation Color Code. The VAL and ACC on 5 February 2008 had been raised to Watch and Orange, respectively, as a result of ash emissions. VHA is volcanic haze advisory. Data from the Emergency Management Office of the Commonwealth of the Mariana Islands, Hawaiian Volcano Observatory, and the Washington Volcanic Ash Advisory Center (VAAC).

A Volcanic Haze Advisory was issued during the week of 20-26 February 2008 for the islands of Tinian, Saipan, and Rota due to high SO2 levels (see map, BGVN 28:04). According to an article on 26 February in the Saipan Tribune (Ferdie de la Torre, reporter), the Emergency Management Office of the Commonwealth of the Northern Mariana Islands (EMO-CNMI) advised people who had breathing problems to remain indoors during the volcanic haze advisory, and reminded mariners to take precautionary measures due to low visibility. The newspaper article indicated that EMO-CNMI's SO2 analyzer (location not reported) measured 161 ppb (parts per billion). According to the article, Gov. Benigno Fitial declared that Anatahan island was still unsafe for human habitation and ordered all travel to the island restricted, with the exception of scientific expeditions.

Starmer (2005) stated that "Ash fallout from the 2003 eruption caused extensive damage to nearshore reef habitats, especially on the northern side. Although all surveyed locations during the 2003 National Oceanic and Atmospheric Administration (NOAA) Marianas Research and Monitoring Program (MARAMP) cruise contained a layer of ash covering the substrate (figure 33), portions of the south shore and southeastern corner had only a veneer layer."

Figure 33. Part of an ash-covered coral reef on a submarine flank of Anatahan island. The reef organisms appear greenish gray in color owing to fine-grained ash. Taken from Starmer (2005).

Reference. Starmer, J. (ed.) 2005, The state of coral reef ecosystems of the Commonwealth of the Northern Mariana Islands (p. 399-441) in Waddell, J.E. (ed.), 2005, The state of coral reef ecosystems of the United States and Pacific Freely Associated States 2005: NOAA Technical Memorandum NOS NCCOS 11, NOAA/NCCOS Center for Coastal Monitoring and Assessment's Biogeography Team, Silver Spring, MD, USA, 522 p.

Our most recent report on Anatahan (BGVN 33:12) discussed sulfur dioxide emissions and steam plumes during 2008. This report covers activity between January and October 2009.

A team of research scientists from the University of Tokyo and Kyushu University visited the volcano during the week of 19 January. They worked with the Emergency Management Office of the Commonwealth of the Northern Mariana Islands (CNMI) to perform seismic station maintenance. The team observed no unusual volcanic phenomena. Seismic levels remained low, and no anomalies were observed in satellite imagery.

The U.S. Geological Survey (USGS) reported that seismic activity at Anatahan during the first half of 2009 was generally at background levels. On 11 February a brief episode of tremor occurred. A low level plume was observed in satellite images on 13 June, but there was no evidence that it contained ash. Nothing unusual was observed in satellite images throughout the rest of the week. According to the USGS, Anatahan was quiet as of 6 November.

Geological Background

The elongate, 9-km-long island of Anatahan in the central Mariana Islands consists of a large stratovolcano with a 2.3 x 5 km, E-W-trending compound summit caldera. The larger western portion of the caldera is 2.3 x 3 km wide, and its western rim forms the island's 790-m high point. Ponded lava flows overlain by pyroclastic deposits fill the floor of the western caldera, whose SW side is cut by a fresh-looking smaller crater. The 2-km-wide eastern portion of the caldera contained a steep-walled inner crater whose floor prior to the 2003 eruption was only 68 m above sea level. A submarine cone, named NE Anatahan, rises to within 460 m of the sea surface on the NE flank, and numerous other submarine vents are found on the NE-to-SE flanks. Sparseness of vegetation on the most recent lava flows had indicated that they were of Holocene age, but the first historical eruption did not occur until May 2003, when a large explosive eruption took place forming a new crater inside the eastern caldera.

This compilation of synonyms and subsidiary features may not be comprehensive. Features are organized into four major categories: Cones, Craters, Domes, and Thermal Features. Synonyms of features appear indented below the primary name. In some cases additional feature type, elevation, or location details are provided.

Cones

Feature Name

Feature Type

Elevation

Latitude

Longitude

NE Anatahan

Submarine cone

-460 m

16° 26' 0" N

145° 47' 0" E

Craters

Feature Name

Feature Type

Elevation

Latitude

Longitude

Southeast crater

Crater

Photo Gallery

Two coalescing volcanoes form the elongate, 9-km-long island of Anatahan in the central Mariana Islands. The rim of a 2.5 x 5 km, E-W-trending summit depression consisting of overlapping summit calderas forms the low point at the center of the island. The floor of the steep-walled inner crater of the younger eastern cone (right) is only 68 m above sea level.

Photo by Dick Moore, 1990 (U.S. Geological Survey).

The first historical eruption of Anatahan volcano is seen in this photograph taken on May 10, 2003 of an ash cloud produced from the eruption that began earlier that day. The cloud top is at ~ 4.6 km and emanates from the eastern crater. The view is toward the SW. Intermittent explosive eruptions took place until June 16. Lava flows were emplaced on the floor of the caldera beginning on June 4 and later coalesced to form a small lava dome in the new 300-m-wide crater.

Photo courtesy of Commonwealth of Northern Mariana Islands, 2003.

The two coalescing volcanoes forming the elongate, 9-km-long island of Anatahan in the central Mariana Islands are apparent in this aerial view from the south. The low point in the center of the island results in part from overlapping 2.3 x 5 km wide calderas, the largest in the Mariana Islands. The larger western caldera is 2.3 x 3 km wide and extends eastward from the 788-m-high summit of the western volcano (left). The volcano's first historical eruption in 2003 took place from a small crater within the 2-km-wide eastern caldera.

Photo courtesy of U. S. Geological Survey, 1994.

An ash-bearing eruption column rises from the east crater of Anatahan on June 16, 2003. The column reached a maximum height of about 2.5 km in less than 40 seconds. This energetic pulse of activity was accompanied by intense tremor and earthquakes as recorded by the single seismic station on the island.

Photo by U.S. Geological Survey, Hawaiian Volcano Observatory, 2003.

A dark ash cloud rises from a vent at the east crater (far right) of Anatahan as seen from a helicopter looking NW on August 24, 2005. Eruptive activity had begun in 2004, when a new lava dome was seen on April 12, along with fresh ejecta at the bottom of the crater. Intermittent explosive activity occurred throughout the year. In 2005 the 2003 crater floor was almost entirely covered by fresh lava out to a diameter of ~ 1 km. Powerful explosive eruptions in April, June, and July 2005 sent eruption plumes to as high as 15 km.

Photo by Takeshi Matsushima, 2005 (Kyushu University).

References

The following references have all been used during the compilation of data for this volcano, it is not a comprehensive bibliography. Discussion of another volcano or eruption (sometimes far from the one that is the subject of the manuscript) may produce a citation that is not at all apparent from the title.

WOVOdat is a database of volcanic unrest; instrumentally and visually recorded changes in seismicity, ground deformation, gas emission, and other parameters from their normal baselines. It is sponsored by the World Organization of Volcano Observatories (WOVO) and presently hosted at the Earth Observatory of Singapore.

EarthChem develops and maintains databases, software, and services that support the preservation, discovery, access and analysis of geochemical data, and facilitate their integration with the broad array of other available earth science parameters. EarthChem is operated by a joint team of disciplinary scientists, data scientists, data managers and information technology developers who are part of the NSF-funded data facility Integrated Earth Data Applications (IEDA). IEDA is a collaborative effort of EarthChem and the Marine Geoscience Data System (MGDS).